Remove obsolete after.gitconfig

dotfiles/gittum/bin/setup

 printf "    path = $tilde/.gittum/host/$host.gitconfig\n" >> ~/.gitconfig
 printf "    path = $tilde/.gittum/user/$user.gitconfig\n" >> ~/.gitconfig
 printf "    path = $tilde/.gittum/private.gitconfig\n" >> ~/.gitconfig
-printf "    path = $tilde/.gittum/after.gitconfig\n" >> ~/.gitconfig