remove any remote tracking branches that no longer exist remotely

[MagiCarbon]

fix the following error:

error: cannot lock ref 'refs/remotes/origin/dev/rindeal': 'refs/remotes/origin/dev' exists; cannot create 'refs/remotes/origin/dev/rindeal'                                                                         
From https://github.com/rindeal/rindeal-ebuild-repo                                                                                                                                                                 
 ! [new branch]      dev/rindeal -> origin/dev/rindeal  (unable to update local ref)                               
error: some local refs could not be updated; try running                                                          
 'git remote prune origin' to remove any old, conflicting branches