Remove now-unused message_cache_key message cache.

Originally this cache was used to transmit data from Django to Tornado
(and also for general message caching purposes), but now nothing
actually reads from this cache, so we can eliminate it.

zerver/lib/event_queue.py

@@ -21,7 +21,7 @@ import random
 import traceback
 from zerver.models import UserProfile, Client
 from zerver.decorator import RespondAsynchronously
-from zerver.lib.cache import cache_get_many, message_cache_key, \
+from zerver.lib.cache import cache_get_many, \
     user_profile_by_id_cache_key, cache_save_user_profile, cache_with_key
 from zerver.lib.handlers import clear_handler_by_id, get_handler_by_id, \
     finish_handler, handler_stats_string