The clear function in CTxMemPool do not clear the member vTxHashes

Summary: The clear function in CTxMemPool do not clear the member vTxHashes

Test Plan: make check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D392