doc: Remove fee delta TODO from txmempool.cpp

Summary:

This refactor request was added in commit eb30666, though it didn't explain why the refactor is needed and what the goal is. Given that this wasn't touched for more than 5 years, it doesn't seem critical. Generally, non-trivial TODOs make more sense as GitHub issues, so that they can be discussed and triaged more easily.

This is a backport of core#23416

Test Plan: NA

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12594