remove unused BlockValidationResult::BLOCK_RECENT_CONSENSUS_CHANGE

Summary:
Nothing returns this status in this codebase, and there is no foreseeable need to keep these unreachable code branches.

Similar to D15014

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15027