Revert "Release Python3.8 wheels for Windows"

tools/internal_ci/helper_scripts/install_python38.ps1

tools/internal_ci/helper_scripts/prepare_build_windows.bat

tools/internal_ci/windows/grpc_build_artifacts.bat

tools/internal_ci/windows/grpc_build_packages.bat

tools/internal_ci/windows/grpc_distribtests.bat

tools/internal_ci/windows/grpc_distribtests_standalone.bat

tools/run_tests/artifacts/artifact_targets.py

tools/run_tests/artifacts/build_artifact_python.bat