[validation interface] Remove vtxConflicted from BlockConnected

Summary:

The wallet now uses TransactionRemovedFromMempool to be notified about
conflicted wallet, and no other clients use vtxConflicted.

This is a backport of Core PR17477 [2/6]

https://github.com/bitcoin/bitcoin/pull/17477/commits/cdb893443cc16edf974f099b8485e04b3db1b1d7

Depends on D8864

See D6653 for differences in validationinterface.cpp

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8865