Remove comments about possible maintenance events during live migration (GoogleCloudPlatform#1906)

The comments do not match up with the published documentation which
states that only the MIGRATE_ON_HOST_MAINTENANCE event will ever be
published for a host to receive.
(https://cloud.google.com/compute/docs/storing-retrieving-metadata#maintenanceevents)

Author: djmailhot

compute/metadata/main.py

@@ -55,9 +55,6 @@ def wait_for_maintenance(callback):
         if r.text == 'NONE':
             maintenance_event = None
         else:
-            # Possible events:
-            #   MIGRATE_ON_HOST_MAINTENANCE: instance will be migrated
-            #   TERMINATE_ON_HOST_MAINTENANCE: instance will be shut down
             maintenance_event = r.text
 
         if maintenance_event != last_maintenance_event: