Remove CTxMemPool params from ATMP

Summary:
Co-authored-by: John Newbery <1063656+jnewbery@users.noreply.github.com>
Co-authored-by: Jon Atack <jon@atack.com>

This is a backport of core#23465

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12457