Drop the distributed lock for FMN's redis caching

By default, locks in redis never expire. This can lead to a deadlock if
a worker dies unexpectedly while trying to create a cache entry, which
leaves the lock in redis for the rest of time. When a worker attempts to
refresh the cache later, it'll wait on the lock permanently. We could
set the "lock_timeout" key in the dogpile.cache configuration, but
realistically we don't need to lock at all. Populating the cache is a
single GET request and we only have a couple workers so if they happen
to try the same entry at the same time, we'll just have 4-5 requests
instead of 1.

roles/notifs/backend/templates/fmn.consumer.py

@@ -92,7 +92,6 @@ config = {
             "port": 6379,
             "db": 0,
             "redis_expiration_time": 60*60*24,   # 1 day
-            "distributed_lock": True
         },
     },