Remove FullGCCount_lock

After the removal of CMS (JDK-8229049) and the introduction of G1OldGCCount_lock (JDK-8232588), the FullGCCount_lock looks vestigial. It is locked and notified by the two overloads of GenCollectedHeap::update_full_collections_completed. The 1-arg overload is no longer called (it was only called by CMS). And there are no waits, so the notification is useless. Since SerialGC is the only collector still using GenCollectedHeap, the locking in the 0-arg overload isn't needed. Doing those cleanups would leave it unused.