Remove unused mutex instance.

Change-Id: I90e33818b79355e47343f5134b93575a1a5aa75c
Reviewed-on: http://review.couchbase.org/29154
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>