Fix syntax with gear unRegisterFunction()

This is to fix the following exception:

  2017-10-30 17:02:44,604 ERROR zuul.ExecutorServer: Exception in governor thread:
  Traceback (most recent call last):
    File "/usr/local/lib/python3.5/dist-packages/zuul/executor/server.py", line 1883, in run_governor
      self.manageLoad()
    File "/usr/local/lib/python3.5/dist-packages/zuul/executor/server.py", line 1897, in manageLoad
      self.unregister_work()
    File "/usr/local/lib/python3.5/dist-packages/zuul/executor/server.py", line 1693, in unregister_work
      self.executor_worker.unregisterFunction("executor:execute")
  AttributeError: 'ExecutorExecuteWorker' object has no attribute 'unregisterFunction'

Change-Id: Ib898c4046b83829bf2a42b92897261956bc249bc
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

zuul/executor/server.py

@@ -1690,7 +1690,7 @@ class ExecutorServer(object):
 
     def unregister_work(self):
         self.accepting_work = False
-        self.executor_worker.unregisterFunction("executor:execute")
+        self.executor_worker.unRegisterFunction("executor:execute")
 
     def stop(self):
         self.log.debug("Stopping")