Remove unnecessary validation in observe_seqno

The check for the length of data is already performed in the
observe_seqno_validator function in memcached. Hence, removing
this check

Change-Id: I4f20a2b50ebc2a6bddf5c44b5a50c106ad75bd18
Reviewed-on: http://review.couchbase.org/44289
Tested-by: Sriram Ganesan <sriram@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>