style-only: No need for std::pair for vSortedByHeight

Summary:
...since the height information in already in CBlockIndex* and we can
use an easy custom sorter.

This is a partial backport of core#24515
https://github.com/bitcoin/bitcoin/pull/24515/commits/42e56d9b188f97c077ed2269e24acc0be35ece17

Also includes a comment removal from https://github.com/bitcoin/bitcoin/pull/24515/commits/3bbb6fea051f4e19bd2448e401a6c4e9b4cc7a41
The rest of this commit is already done in Bitcoin ABC (review of D12993 and D12994)

Depends on D13020

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13021