sagemathgh-39284: Remove failing modular ci tests
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

As suggested at
sagemath#39206 (comment).

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#39284
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik

Author: Release Manager

.github/workflows/build.yml

@@ -29,7 +29,7 @@ concurrency:
 #
 # The three workflows:
 #
-#  - build.yml (with jobs test-new, test-mod, test-long),
+#  - build.yml (with jobs test-new, test-long),
 #  - doc-build.yml,
 #  - doc-build-pdf.yml
 #
@@ -50,7 +50,7 @@ concurrency:
 # This baseline is transparently improved by our use of the GH Actions cache,
 # see https://docs.docker.com/build/ci/github-actions/cache/#cache-backend-api.
 #
-# Jobs test-mod and test-long are only started after test-new completed;
+# Jobs test-long is only started after test-new completed;
 # but the workflows doc-build.yml and doc-build-pdf.yml are started independently.
 #
 #  - When nothing is cached and the 3 workflows are launched in parallel,
@@ -254,104 +254,6 @@ jobs:
           ./sage -t --long --format github -p4 ${{ steps.changed-files.outputs.doctests_all_changed_files }}
         shell: sh .ci/docker-exec-script.sh BUILD /sage {0}
 
-  test-mod:
-    runs-on: ubuntu-latest
-    needs: [test-new]
-    services:
-      # https://docs.docker.com/build/ci/github-actions/local-registry/
-      registry:
-        image: registry:2
-        ports:
-          - 5000:5000
-    strategy:
-      fail-fast: false
-      matrix:
-        targets:
-          - sagemath_categories-check
-    steps:
-      - name: Maximize build disk space
-        uses: easimon/maximize-build-space@v10
-        with:
-          # need space in /var for Docker images
-          root-reserve-mb:      30000
-          remove-dotnet:        true
-          remove-android:       true
-          remove-haskell:       true
-          remove-codeql:        true
-          remove-docker-images: true
-
-      - name: Checkout
-        id: checkout
-        uses: actions/checkout@v4
-
-      - name: Install test prerequisites
-        # From docker.yml
-        run: |
-          sudo DEBIAN_FRONTEND=noninteractive apt-get update
-          sudo DEBIAN_FRONTEND=noninteractive apt-get install tox
-          sudo apt-get clean
-          df -h
-
-      - name: Merge CI fixes from sagemath/sage
-        # From docker.yml
-        # This step needs to happen after the commit sha is put in DOCKER_TAG
-        # so that multi-stage builds can work correctly.
-        run: |
-          .ci/merge-fixes.sh
-        env:
-          GH_TOKEN: ${{ github.token }}
-
-      # Building
-
-      - name: Generate Dockerfile
-        # From docker.yml
-        run: |
-          tox -e ${{ env.TOX_ENV }}
-          cp .tox/${{ env.TOX_ENV }}/Dockerfile .
-        env:
-          # Only generate the Dockerfile, do not run 'docker build' here
-          DOCKER_TARGETS: ""
-
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-        with:
-          driver-opts: network=host
-
-      - name: Build Docker image
-        id: image
-        uses: docker/build-push-action@v6
-        with:
-          push:       true
-          load:       false
-          context:    .
-          tags:       ${{ env.BUILD_IMAGE }}
-          target:     with-targets
-          cache-from: type=gha
-          cache-to:   type=gha,mode=max
-          build-args: |
-            NUMPROC=6
-            USE_MAKEFLAGS=-k V=0 SAGE_NUM_THREADS=4 --output-sync=recurse
-            TARGETS_PRE=build/make/Makefile
-            TARGETS=${{ needs.test-new.outputs.build_targets }}
-
-      - name: Start container
-        id: container
-        if: (success() || failure())
-        run: |
-          docker run --name BUILD -dit \
-                     --mount type=bind,src=$(pwd),dst=$(pwd) \
-                     --workdir $(pwd) \
-                     ${{ env.BUILD_IMAGE }} /bin/sh
-
-      # Testing
-
-      - name: Test modularized distributions
-        if: (success() || failure()) && steps.container.outcome == 'success'
-        run: |
-          export MAKE="make -j2 --output-sync=recurse" SAGE_NUM_THREADS=4
-          make V=0 tox-ensure && make ${{ matrix.targets }}
-        shell: sh .ci/docker-exec-script.sh BUILD /sage {0}
-
   test-long:
     runs-on: ubuntu-latest
     needs: [test-new]