xrow_header_has_key() isn't used anywhere.

The purpose of iproto_dml_body_has_key() is unclear - we use it to skip
unknown DML keys, but we would skip them anyway in the switch-case down
the code path, like we do while decoding other requests. The only useful
thing it does is implicitly checks that the key has type MP_UINT, but we
can do that explicitly instead.