Remove an unused variable in rollback unit test

Remove an unused variable since it is violating the fact
that couch_file process pid can change in between updates since
viewgroup cleaner resets the couch file process for the viewgroup.

Change-Id: I8e3f9020e769c195970ae2984ea4bcaf2d5afee6
Reviewed-on: http://review.couchbase.org/31040
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>