Remove asyncio mark from non-async test function

Author: vchudnov-g

tests/asyncio/test_operation_async.py

@@ -84,7 +84,6 @@ async def test_constructor():
     assert await future.running()
 
 
-@pytest.mark.asyncio
 def test_metadata():
     expected_metadata = struct_pb2.Struct()
     future, _, _ = make_operation_future(