Remove extraneous rowid parameter from CouchKVStore::get()

Change-Id: Ic281ae03265ac900776bbce2b59c92918060c383
Reviewed-on: http://review.couchbase.org/43346
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Sriram Ganesan <sriram@couchbase.com>