validation: Remove review-only comments + assertions

Summary:
[META] This is a followup to "validation: Move FindFilesToPrune{,Manual} to BlockManager" removing comments and assertions meant only to show that the change is correct.

This is a backport of core#19927 [6/6]
https://github.com/bitcoin/bitcoin/pull/19927/commits/72a1d5c6f3834e206719ee5121df7727aed5b786

Depends on D9824

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9825