remove unused rpc import in modelcheckpoint causing import error (#5198)

* remove unused rpc import

* isort

Co-authored-by: Jirka Borovec <[email protected]>

Author: awaelchli

pytorch_lightning/callbacks/model_checkpoint.py

@@ -34,7 +34,6 @@
 from pytorch_lightning import _logger as log
 from pytorch_lightning.callbacks.base import Callback
 from pytorch_lightning.metrics.metric import Metric
-from pytorch_lightning.plugins.rpc_plugin import RPCPlugin
 from pytorch_lightning.utilities import rank_zero_info, rank_zero_only, rank_zero_warn
 from pytorch_lightning.utilities.cloud_io import get_filesystem
 from pytorch_lightning.utilities.exceptions import MisconfigurationException