Drop wgFlowCacheKey from CommonSettings.php

We have a patch going live this thursday that changes the
representation of UUID's within the cache.  For this to
work without issues the old and the new need to be on separate
cache versions(as defined in their Flow.php files).

Change-Id: Id1f2e0acf2cb1b8dffce0d9c39bdce3477f2e21f