[mempool] simplify some check() logic

Summary:
No transaction in the mempool should ever be a coinbase.

Since mempoolDuplicate's backend is the chainstate coins view, it should
always contain the coins available.

This is a backport of core#23157 [7/8]
https://github.com/bitcoin/bitcoin/pull/23157/commits/ed6115f1eae0eb4669601106a9aaff078a2f3a74

Depends on D12172

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12173