Remove unneeded function getMutationIdByKey()

This function was only used by our testing framework and since each
queued_item instance contains the mutation id (by seqno) we no
longer need this function in order to get this information.

Change-Id: I8e1fbe31380c0cc1837342fe3ac9e65d570a116f
Reviewed-on: http://review.couchbase.org/41508
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>