[mempool] Remove NotifyEntryAdded and NotifyEntryRemoved callbacks

Summary:

NotifyEntryAdded never had any subscribers so can be removed.

Since ConnectTrace no longer subscribes to NotifyEntryRemoved, there are
now no subscribers.

The CValidationInterface TransactionAddedToMempool and
TransactionRemovedFromMempool methods can now provide this
functionality. There's no need for a special notifications framework for
the mempool.

This concludes backport of Core PR17477 [6/6]
https://github.com/bitcoin/bitcoin/pull/17477/commits/e57980b4738c10344baf136de3e050a3cb958ca5
Depends on D8867

Note: See D6451 for why NotifyEntryRemoved has to be removed twice. It was probably duplicated by mistake.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8869