Revert "environment tests: clear the singularity image cache"

This reverts commit d13ddc4.

Author: mr-c

tests/test_environment.py

@@ -9,7 +9,7 @@
 import pytest
 from packaging.version import Version
 
-from cwltool.singularity import _IMAGES, _IMAGES_LOCK, get_version
+from cwltool.singularity import get_version
 
 from .util import env_accepts_null, get_tool_env, needs_docker, needs_singularity
 
@@ -22,12 +22,6 @@
 EnvChecks = dict[str, CheckerTypes]
 
 
-@pytest.fixture(autouse=True)
-def clear_singularity_image_cache() -> None:
-    with _IMAGES_LOCK:
-        _IMAGES.clear()
-
-
 def assert_envvar_matches(check: CheckerTypes, k: str, env: Mapping[str, str]) -> None:
     """Assert that the check is satisfied by the key in the env."""
     if check is None: