Disable github rate limit logging

We are currently well under our rate limits for github and logging the
rate limit adds a number of requests to github for each github event
processed. This can be reenabled if we suspect rate limit based
failures, but until then let's reduce the number of requests and
simplify our logging.

Change-Id: I02878f9749737d297680cd324d6dafeefc44a005

hiera/group/zuul-scheduler.yaml

@@ -28,6 +28,7 @@ zuul_connections:
   - name: 'github'
     driver: 'github'
     app_key: '/etc/zuul/github.key'
+    rate_limit_logging: 'false'
 
   - name: 'googlesource'
     driver: 'git'