CLC-3187-ii: LiveUpdates: eliminate MERGED_FEEDS_EXPIRATION

https://jira.media.berkeley.edu/jira/browse/CLC-3187

Deleted MERGED_FEEDS_EXPIRATION instance of UserCacheInvalidator.

Since UserCacheWarmer now forces write to the cache, there's no
need to expire the cache on merged feeds via observers.

Also deleted MERGED_FEEDS_EXPIRATION from the cache
cache_tools initializer and references to it in the
live_updates_warmer_spec.