init: Remove redundant -*mempool* queries

Summary:
Now that MemPoolOptions has a correctly-determined max_size member, perform sanity checks on that instead of re-determining the
options.

This concludes backport of core#25290
https://github.com/bitcoin/bitcoin/pull/25290/commits/d1684beabe5b738c2cc83de83e1aaef11a761b69

Depends on D15301

Note that some of the commits of this PR were not applicable, because they related to fee estimation or ancestor/descendant limits.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15302