Special:BannerLoader: test campaign staleness

This complements the client-side test for campaign staleness. The goal
is to avoid banners displaying due to campaings that have ended. This
may happen because of eternally cached JS.

This patch also cleans up a bit of dead code.

Bug: T88276
Change-Id: I35fcac8143488786746cea28eba4f663c3cb40b1