Do not include app and config timestamps in Last-Modified header

MediaWiki assumes timestamps are in TS_MW format when calculating value for ``Last-Modified`` response header. This hook was passing UNIX timestamps, making no changes to last modified value. Removing.

Reverts 67396db33d7a457a043c4e388b166b64fd37c1f1

@michalroszka / @prein / @wladekb