conf: remove unused (and racy) migration

Created by: slimsag

This migration existed to merge the critical configuration into the site configuration as we remoed the Management Console in v3.11.

The migration is no longer needed as anyone upgrading will have already ran this OR will need to visit 3.11 first per our upgrade guidelines, and additionally this migration is racey when specifying site or critical configuration via a file as both get applied at te same time which leads to ones' edits being rejected. In practice, this is not an issue as the migration is eventually successful in any case -- it just means the server may restart needlessly due to "the migration failing".

Fixes #9019