The only use of the FreeIdSet class is by DirtyCardQueueSet, in mut_process_buffer.

Only one of the DirtyCardQueueSet objects actually uses this code, the one returned by JavaThread::dirty_card_queue_set(). Other sets allocate but never use their FreeIdSet. We should avoid wasting space for the unused sets.

The FreeIdSet class could be moved into dirtyCardQueue.cpp, since it isn't needed anywhere else; just leave a forward reference behind for the member type in DirtyCardQueueSet.