[validation] Remove NotifyEntryRemoved callback from ConnectTrace

Summary:

ConnectTrace used to subscribe to the mempool's NotifyEntryRemoved
callback to be notified of transactions removed for conflict. Since
PerBlockConnectTrace no longer tracks conflicted transactions,
ConnectTrace no longer requires these notifications.

This is a backport of Core PR17477 [4 & 5/6]
https://github.com/bitcoin/bitcoin/pull/17477/commits/969b65f3f527631ede1a31c7855151e5c5d91f8f
https://github.com/bitcoin/bitcoin/pull/17477/commits/2dd561f36124972d2364f941de9c3417c65f05b6

Backport note: these 2 commits have been squashed because the first one left an unused private attribute 'pool', causing a warning / error for build-clang

Depends on D8866

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8867