Remove Instance Group concept/usage from WorkflowJobs

This also relaxes some of the task manager rules on Instance Groups
down the full stack such that workflow jobs tend to shortcut the
processing or omit it altogether.

This lets the workflow job spawning logic exist outside of the
instance group queues, which it doesn't need to participate in in the
first place.

awx/main/models/workflow.py

@@ -474,7 +474,7 @@ class WorkflowJob(UnifiedJob, WorkflowJobOptions, SurveyJobMixin, JobNotificatio
 
     @property
     def preferred_instance_groups(self):
-        return self.global_instance_groups
+        return []
 
     '''
     A WorkflowJob is a virtual job. It doesn't result in a celery task.