remove unused BlockValidationResult::BLOCK_FINALIZED

Summary:
This enum value was introduced in D6698 and stopped being used sometime between D6708 and the present day. I can't tell exactly when.

Removing it will let me backport core#24304 without having to add an unreachable case BlockValidationResult::BLOCK_FINALIZED: in the switch block.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15014