chore(generator): revert noxfile.py mypy command and restore test_generator assertions

Author: hebaalazzeh

packages/gapic-generator/noxfile.py

@@ -618,13 +618,7 @@ def showcase_mypy(
         session.chdir(lib)
 
         # Run the tests.
-        session.run(
-            "mypy",
-            f"--config-file={MYPY_CONFIG_FILE}",
-            "-p",
-            "google",
-            "--check-untyped-defs",
-        )
+        session.run("mypy", "-p", "google", "--check-untyped-defs")
 
 
 @nox.session(python=NEWEST_PYTHON)