Remove files that have been moved to astropy-helpers

We don't necessarily have to merge this for 1.0 - this hasn't been decided for sure. But for now, I thought I'd do this to make sure that it all works fine without those files in case any other changes are required.

Do we want to maintain the backward-compatibility for 1.0? This is 6 months away, so the question is whether all affiliated packages that have already had releases will have had a release by then that leverages astropy-helpers (or at least doesn't leverage the old setup helpers).

cc various developers and affiliated package maintainers: @eteq @mdboom @embray @cdeil @keflavich @ejeschke @wkerzendorf @RayPlante @mwcraig @crawfordsm @kbarbary @sophiathl

If anyone needs help transitioning to the new system, please let me know and I can help.