Fire TransactionRemovedFromMempool from mempool

Summary:
This commit fires TransactionRemovedFromMempool callbacks from the
mempool and cleans up a bunch of code.

This is a backport of Core PR14384

policyestimator_tests was modified to use TestingSetup because it uses the CTxMemPool::TrimToSize function which now may trigger the signal, but the signals handlers are not setup properly to do so using BasicTestingSetup.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6451