Drop support for Home and XDG config files

This has been a huge support burden for us. I seriously considered doing
this in 3.0 but caved to a vocal minority and the desire to keep as much
backwards compatibility as possible. At this point, however, I'm done
witnessing the abuse Anthony has to suffer over this and I'm done with
the undue hostility that people who don't bother to read the docs
display. Hopefully, this eases that a bit.