http session replication produces WARN message about null session

Currently when a session is removed explicitly (e.g., invalidated), the Tomcat background thread will kick in periodically to search for expired session. And this WANR message will be produced:

log_.warn("Has session id: " +ids.get + " but session is null. Will remove this from map");

This is actually a harmless message but it is annoying. What happened was that when session was removed from the cache store, the fqn (of which is the session id) still exists, albeit it is empty.

Solution is just to remove the whole thing. Will do this to 3.2, 4.0 and head.