[wallet] Notify conflicted transactions in TransactionRemovedFromMempool

Summary:

The only CValidationInterface client that cares about transactions that
are removed from the mempool because of CONFLICT is the wallet.

Start using the TransactionRemovedFromMempool method to notify about
conflicted transactions instead of using the vtxConflicted vector in
BlockConnected.

This is a backport of Core PR17477 [1/6]
https://github.com/bitcoin/bitcoin/pull/17477/commits/1168394d759b13af68acec6d5bfa04aaa24561f8

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8864