Don't run postfetch_post_update for a DELETE

Don't run postfetch_post_update for a DELETE

Fixed 1.2 regression caused by :ticket:`3472` where the handling of an
"updated_at" style column within the context of a post-update operation
would also occur for a row that is to be deleted following the update,
meaning both that a column with a Python-side value generator would show
the now-deleted value that was emitted for the UPDATE before the DELETE
(which was not the previous behavor), as well as that a SQL- emitted value
generator would have the attribute expired, meaning the previous value
would be unreachable due to the row having been deleted and the object
detached from the session.The "postfetch" logic that was added as part of
:ticket:`3472` is now skipped entirely for an object that ultimately is to
be deleted.

Fixes: #4327
Change-Id: Ieac845348979df296bcf7e785c0353bdc6074220