validation: Pass tx pool reference into CheckSequenceLocks
CheckSequenceLocks is called from ATMP and the member function CTxMemPool::removeForReorg without passing in the tx pool object that is used in those function's scope and instead using the global ::mempool instance.
Backport of Bitcoin Core PR13783
Reviewers: Fabien, #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4178