Cleanup branches

[bsipocz]

There are quite a few outdated branches, we should delete next at minimum, but also all the others.

(and IMO should consider following the fork based development model thus not distinguishing between contributors with write access vs those without).

[choldgraf]

+1 from me!

[bsipocz]

I've deleted the ones with an associated merged or closed PR. I think next should go, too, but it's protected so I can't delete it. And of course a bunch of other branches could be cleaned, too.