[validation] Remove conflictedTxs from PerBlockConnectTrace

Summary:

Since we don't add a vtxConflicted vector to BlockConnected the
conflictedTxs member of PerBlockConnectTrace is no longer used.

This is a backport of Core PR17477 [3/6]
https://github.com/bitcoin/bitcoin/pull/17477/commits/5613f9842b4000fed088b8cf7b99674c328d15e1
Depends on D8865

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8866