remove coinbase spend tracking in CTxMemPoolEntry

Summary:
Tracking of coinbase spends in CTxMemPoolEntry was introduced in https://github.com/bitcoin/bitcoin/pull/6915/commits/7e49f5f8b4e237d7212d027a7bea4bbd52c9e7b6
After removing removeForReorg in D13133, a lot of the code that previously cared about whether a mempool tx spends a coinbase no longer does.

Coinbase maturity is still checked via MemPoolAccept::PreChecks -> Consensus::CheckTxInputs.

Depends on D15788

Test Plan: ninja all check-all bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15789