Remove mempool txs in reverse topological order

Summary:
If a mempool transaction has children, and this transaction gets removed, the node doesn't remove the mempool children in reverse topological order.

Fix this and check Chronik handles the validation queue as expected.

Test Plan: ./test/functional/test_runner.py chronik_mempool_conflicts

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15159