Merge #10684: Remove no longer used mempool.exists(outpoint)

Summary:
22378ad Remove no longer used mempool.exists(outpoint) (Alex Morcos)

Tree-SHA512: e98ffdf9039a1bdd84a388dc2da43221b8850bab413dffbd18c20fb06c791ff364f824d1b3b7c0a696987ae126e52b4ee200bb63c5c46e9782c1973c6d888d32

Backport of Core PR10884
https://github.com/bitcoin/bitcoin/pull/10684/files

Depends on D3620

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, markblundeberg, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3621