Remove the unused TX_RECENT_CONSENSUS_CHANGE tx validation result

Summary: This has been introduced as a placeholder and never used for years. It makes also the misbehaving function for bad tx a bit cleaner as it's not obvious we would not ban a peer for a consensus failure in a hypothetical future.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15311