MB-5307 Don't set vbucket state to dead before deleting.

Several reasons here. First, we did it incorrectly anyway. Second,
ns_memcached:delete_vbucket does it for us. And finally, we hope that with
ep-engine support we'll get rid of this logic altogether.

Change-Id: I45874c5f59d637631ab8e93b0536d04631c279fc
Reviewed-on: http://review.couchbase.org/25235
Reviewed-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>