assumeutxo: Remove BLOCK_ASSUMED_VALID flag

Summary:
Flag adds complexity and is not currently used for anything.

This is a partial backport of core#29370
https://github.com/bitcoin/bitcoin/pull/29370/commits/9d9a7458a2570f7db56ab626b22010591089c312

Depends on D17873

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17874