Remove stop method from reporters, sources, triggers

It still remains in the drivers that the connections utilize.

Change-Id: Ie6efb57af297fbd546eed3e1104299b2e1a5205e

zuul/reporter/__init__.py

@@ -33,9 +33,6 @@ class BaseReporter(object):
     def setAction(self, action):
         self._action = action
 
-    def stop(self):
-        """Stop the reporter."""
-
     @abc.abstractmethod
     def report(self, source, pipeline, item):
         """Send the compiled report message."""