validation: Pass tx pool reference into CheckSequenceLocks

Summary:
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
https://github.com/bitcoin/bitcoin/pull/13783

Test Plan:

make check-all

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4178