Remove `isValid` from transaction deserialization

[lehins]

#5480 has implemented part of CIP-167, namely:

• it prevents isValid=False from being decoded
• Preserves backwards compatibility for easier transition from Conway to Dijkstra era by allowing both: presence of isValid=True and adding new functionality of no flag at all.

This ticket is about removing this backwards compatibility of this false flag being present, namely removal of this case in CDDL:

transaction_body, transaction_witness_set, true, auxiliary_data/ nil