Remove multi_gpu from learn test

ml-agents/mlagents/trainers/tests/test_learn.py

     assert opt.docker_target_name is None
     assert opt.no_graphics is False
     assert opt.debug is False
-    assert opt.multi_gpu is False
     assert opt.env_args is None
 
     full_args = [
     assert opt.docker_target_name == "mydockertarget"
     assert opt.no_graphics is True
     assert opt.debug is True
-    assert opt.multi_gpu is True
 
 
 @patch("builtins.open", new_callable=mock_open, read_data="{}")