cluster/distribute: Remove suprious fd_unref call

After fix http://review.gluster.org/4282 (libglusterfsterfs/syncop: do not
hold ref on the fd in cbk) was pushed, syncop_open does not take a ref anymore.

Change-Id: I5543986a4f2d965eef42ca979b39ac75439cec49
BUG: 910661
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4509
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>