remove unused code

We use `/includes/cache/MemcachedSessions.php` and
`/includes/cache/wikia/MemcachedSessions.php` is not even included
anywhere.
log UserLogin related events

UserLogin related events in UserLoginSpecialController::login() are
logged to Elasticsearch.
log session_write_close calls

All session_write_close() calls are logged to Elasticsearch.
log ExternalUser related events

ExternalUser related events in ExternalUser_Wikia::updateUse(),
ExternalUser_Wikia::addToDatabase() and
ExternalUser_Wikia::authenticate() are logged to Elasticsearch.
fix whitespace
Merge pull request #5244 from Wikia/PLATFORM-508-helios-logging-auth-events

Platform 508 helios logging auth events