Remove unnecessary task mutex from BGFetcher.

Acccessing the GIO APIs from the BGFetcher is already
synchronized by the lock inside the GIO.

Change-Id: I5309bc152aa52aa731cfc392df773bbaf5ef2a7b
Reviewed-on: http://review.couchbase.org/29180
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>