clean up MemPoolAccept aliases

Summary:

The aliases are leftover from a previous MOVEONLY refactor - they are
unnecessary and removing them reduces the diff for splitting out mempool
Checks from PreChecks, making RBF variables MemPoolAccept-wide, etc.

This is a partial backport of core#23381
https://github.com/bitcoin/bitcoin/pull/23381/commits/9e910d8152e08d26ecce6592870adbe5dabd159e

Depends on D12352

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12359