MB-100 do not crash ebucketmigrator_srv child after Ctrl-C

- removes unnecessary crash report. in this case all we might
  need to know is that the socket is closed

- the current behavior depends on a race. if memcached
  shuts down before gen_tcp:send, there will be crash report.
  if a moment later, then there will be just a warning.

Change-Id: I988803677cf530a4b67f6ec0ca217e34f05abaf1
Reviewed-on: http://review.couchbase.org/29211
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>