remove: enable command for stopping workflows (#6514)

* A documented use case for `cylc remove` is "orphaning" stuck
  submissions, e.g. jobs that we can no longer poll or kill due to
  platform or network issues.
* It is conceivable that we might need to do this whilst a workflow is
  shutting down.

cylc/flow/network/schema.py

@@ -2161,7 +2161,7 @@ class Meta:
         description = sstrip('''
             Remove one or more task instances from a running workflow.
 
-            Valid for: paused, running workflows.
+            Valid for: paused, running, stopping workflows.
         ''')
         resolver = partial(mutator, command='remove_tasks')