fix: do not set BidiRpc to None when closing it (#698)

Author: Linchin

packages/google-cloud-bigquery-storage/google/cloud/bigquery_storage_v1/writer.py

@@ -308,7 +308,6 @@ def _shutdown(self, reason: Optional[Exception] = None):
 
             if self._rpc is not None:
                 self._rpc.close()
-            self._rpc = None
             self._closed = True
             _LOGGER.debug("Finished stopping manager.")