Remove the obsolete mempool and wallet chained tx limit options

Summary: They are no longer relevant after wellington. Note that they don't deserve a release note because they were debug/test options only.

Test Plan:

ninja check-extended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13951