repo.py: Remove origin remote prefix from branch name

Originally we only worked with the branch name and didn't care if it was found in the local repo or on the remote. Since commit 545056bb ("add ability to use slashes in branch names") we keep the remote prefix (origin/) in the branch name.

This leads for example to parameters like branch=origin/master in the repo.conf when using meta-bump. Actually we don't care about the remote prefix and having it in the repo.conf breaks other parts of the init script.

Therefore let's just strip the prefix and restore the original behavior.