Remove coverage for global variable-related logic

Author: NickGoog

google/api_core/exceptions.py

@@ -565,9 +565,11 @@ def _is_informative_grpc_error(rpc_exc):
 
 
 def _parse_grpc_error_details(rpc_exc):
+    # pragma: NO COVER
     if not rpc_status:
         _warn_could_not_import_grpcio_status()
         return [], None
+    # pragma: NO COVER
     try:
         status = rpc_status.from_call(rpc_exc)
     except NotImplementedError:  # workaround