[Chronik] Refactor: Change to Mempool::remove_mined

Summary:
Change removed_mined_txs to remove_mined, which only removes one tx but returns it.

Also harmonizes the name with the other remove_mined method (and fixes "removed" -> "remove").

Test Plan: ninja check-crates && ninja check-functional

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13779