Add a CValidationInterface::TransactionRemovedFromMempool

Summary:
This is currently unused, but will by used by wallet to cache when
transactions are in the mempool, obviating the need for calls to
mempool from CWalletTx::InMempool()

This is extracted from Core's PR10286 commit a7d3936de8418522dbb161bfef31c234fc6c2503

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2263