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.

BUG: 910661
Change-Id: Idedff91270966e6e70e71ee83785c0228e238d31
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4608
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>