box: don't apply space upgrade func to DML result except for DELETE

There's no need to apply a space upgrade function to the tuple returned
by INSERT, REPLACE, UPDATE, or UPSERT, because it must match the new
format anyway. We only need to process tuples returned by DELETE
requests. That being said, let's move the result processing from
box_process_rw() to memtx_space_execute_delete(). Note, we don't need to
patch vinyl_space_execute_delete(), because in Vinyl DELETE doesn't
return the deleted tuple.

Follow-up commit 21e2def9 ("box: introduce result processor").

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring