Remove constant LocalChannel.hostname attribute (#3687)

This is part of moving LocalChannel to have no state, part of #3515

This hostname and exception will be removed when more #3515 work makes
channel push/pull of files go away.

## Type of change

- Code maintenance/cleanup

parsl/channels/local/local.py

@@ -21,7 +21,6 @@ def __init__(self):
         KwArgs:
             - script_dir (string): Directory to place scripts
         '''
-        self.hostname = "localhost"
         self.script_dir = None
 
     def execute_wait(self, cmd, walltime=None):
@@ -82,7 +81,7 @@ def push_file(self, source, dest_dir):
                 os.chmod(local_dest, 0o700)
 
             except OSError as e:
-                raise FileCopyException(e, self.hostname)
+                raise FileCopyException(e, "localhost")
 
         else:
             os.chmod(local_dest, 0o700)