cluster/dht: Don't do extra unref in dht-migration checks

Problem:
syncop_open used to perform a ref in syncop_open_cbk so the extra
unref was needed but now syncop_open_cbk does not take a ref so no
need to do extra unref.

Fix:
remove the extra fd_unref and let dht_local_wipe do the final unref.

Change-Id: Ibe8f9a678d456a0c7bff175306068b5cd297ecc4
BUG: 961615
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4974
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>