Remove stop method from reporters, sources, triggers

It still remains in the drivers that the connections utilize.

Change-Id: Ie6efb57af297fbd546eed3e1104299b2e1a5205e

zuul/trigger/__init__.py

@@ -28,9 +28,6 @@ class BaseTrigger(object):
         self.sched = sched
         self.connection = connection
 
-    def stop(self):
-        """Stop the trigger."""
-
     @abc.abstractmethod
     def getEventFilters(self, trigger_conf):
         """Return a list of EventFilter's for the scheduler to match against.