auto-prune of shadow topics triggered by prune_all_forums updates wrong timestamp

In response to PHPBB3-14812 the prune_all_forums cron task was changed to also prune shadow topics.

This calls auto_prune() with prune mode 'shadow'. However that function only updates the 'prune_next' row field and not 'prune_shadow_next' resulting in the shadow topic prune task being run on every invocation of the prune_all_forums cron task and spamming the admin log with "auto-pruned forum" messages.

Workaround: disable pruning of shadow topics.