remove MemPoolRemovalReason::REPLACED

Summary: This RBF related removal reason doesn't actually happen in Bitcoin ABC. It is used only as a dummy value in tests, but in the context of these tests the reason doesn't really matter so we can also use another one.

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16084