Revert "Change default for region selection after split on Mixbus"

This shouldn't be necessary: Mixbus should probably just have a different
value for the default in their source tree instead.

This (partially) reverts commit 631467f0bb2a2756c4216738ded804663b44a2a1:
I now realise that I accidentally rolled another fix (missing "region-" in
config variable name) in there too: sorry about that.