fix: remove unused httpx dependency

httpx was accidentally added in GoogleCloudPlatform#364 but is not used anywhere in the codebase.
Removing it reduces our dependency count from 28 to 20 packages.

Impact:
- Removes 8 unnecessary packages (httpx + 7 transitive deps)
- Reduces installation size by ~2MB
- Keeps only the actually needed async dependencies

Author: taeold

pyproject.toml

@@ -29,7 +29,6 @@ dependencies = [
   "gunicorn>=22.0.0; platform_system!='Windows'",
   "cloudevents>=1.2.0,<=1.11.0",                  # Must support python 3.7
   "Werkzeug>=0.14,<4.0.0",
-  "httpx>=0.24.1",
   "starlette>=0.37.0,<1.0.0; python_version>='3.8'",
   "uvicorn>=0.18.0,<1.0.0; python_version>='3.8'",
   "uvicorn-worker>=0.2.0,<1.0.0; python_version>='3.8'",