More cleanup

[matsen]

I don't think that we need normalize_sub_probs anymore

Also, this is somewhat out of order and we could give a plural to mask

       for nt_parent, mask, nt_rates, branch_length, nt_csps in zip(
            self.nt_parents,
            self.mask,
            self.nt_ratess,
            self._branch_lengths,
            self.nt_cspss,