CBSE-473: Set right argument for delete ops

Memcached protocol requires no value for delete operation. As a result,
memcached server will treat it as invalid parameter if value is provided
for delete.

Change-Id: I5f5d80129bcb705b6a9a01616a55b06be532a36a
Reviewed-on: http://review.couchbase.org/25291
Tested-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Jin Lim <jin@couchbase.com>