Remove compdefs for `git branch` and `git remote`

I was wondering why the completion for `git branch` was outdated (it
knows about `--set-upstream` only, and not `-u` etc), and found these
files.

I have not compared them to Zsh's upstream version, but in case there
are any additional features/fixes, they should get forwarded to them.

@Stibbons 
Any objections? - asking you, since you've added them.