Remove channel closing at shutdown (#3685)

LocalChannel, the only extant channel, does not have any close()
behaviour.

This is part of #3515 channel removal.

## Type of change

- Code maintenance/cleanup

parsl/channels/base.py

@@ -81,12 +81,6 @@ def pull_file(self, remote_source: str, local_dir: str) -> str:
         '''
         pass
 
-    @abstractmethod
-    def close(self) -> None:
-        ''' Closes the channel.
-        '''
-        pass
-
     @abstractmethod
     def makedirs(self, path: str, mode: int = 0o511, exist_ok: bool = False) -> None:
         """Create a directory.

parsl/channels/local/local.py

@@ -92,11 +92,6 @@ def push_file(self, source, dest_dir):
     def pull_file(self, remote_source, local_dir):
         return self.push_file(remote_source, local_dir)
 
-    def close(self) -> None:
-        ''' There's nothing to close here, and so this doesn't do anything
-        '''
-        pass
-
     def isdir(self, path):
         """Return true if the path refers to an existing directory.
 

parsl/dataflow/dflow.py

@@ -1268,17 +1268,6 @@ def cleanup(self) -> None:
             executor.shutdown()
             logger.info(f"Shut down executor {executor.label}")
 
-            if hasattr(executor, 'provider'):
-                if hasattr(executor.provider, 'script_dir'):
-                    logger.info(f"Closing channel for {executor.label}")
-
-                    assert hasattr(executor.provider, 'channel'), "Provider with .script_dir must have .channel"
-                    logger.info(f"Closing channel {executor.provider.channel}")
-                    executor.provider.channel.close()
-                    logger.info(f"Closed channel {executor.provider.channel}")
-
-                    logger.info(f"Closed executor channel for {executor.label}")
-
         logger.info("Terminated executors")
         self.time_completed = datetime.datetime.now()