Remove premature attempt to determine alignment group

While trying to understand something else, I realised that this block contributes nothing, because the 'extra' field here is (confusingly) unrelated to the 'extra' expressions in the signature dicts. It refers to a potential suffix after the numerical threshold at the start of a leg ([multiplicity][signature][threshold][extra] a la '3mu20i'). The alignment group is in any case determined later, so chain behaviours remained correct.

For future work, when the information is actually used again (as proposed in ATR-23250), it would be sensible to rename this field and avoid ambiguity.