zshrc: some fixes to make grml-gen-zshrefcard happy again

Thanks: Frank Terbeck <ft@grml.org> for helping
zshrc: drop deprecated grml-rebuildfstab alias

We don't ship [grml-]rebuildfstab since quite some time,
so no point in providing that.
zshrc: some further tuning to make grml-gen-zshrefcard output nice