Use array delete for ids in CouchKVStore saveDocs function

Change-Id: Id5caf22db29e334ff15d05260a95cbf1008aeef7
Reviewed-on: http://review.couchbase.org/34865
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>