indexes, refactor: Remove CBlockIndex* uses in coinstatsindex LookUpOne function

Summary:
This commit does not change behavior in any way.
This is a partial backport of core#25494
https://github.com/bitcoin/bitcoin/pull/25494/commits/addb4f2af183a25ce4a6b6485b5b49575a2ba31b

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17721