net_processing: remove mapRelay, relay txs from m_most_recent_block

Summary:

mapRelay (used to relay announced transactions that are no longer in the mempool) doesn't have an absolute memory limit, only an implicit one based on the rate of transaction announcements.
Remove it.

Note that we can't remove a transaction from the mempool with a replacement transaction, as the sendrawtransaction(tx_b) RPC would fail with txn-mempool-conflict, but as of D15279 we can use avalanche to reject a tx from the mempool.

This is a backport of core#27625
Depends on D18366

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18367