[backport#16400] refactor: Rewrite AcceptToMemoryPoolWorker() using smaller parts

Summary:
4a87c5cfdf7dd72d999ebeaf17db6695a7c6298d [refactor] Rewrite AcceptToMemoryPoolWorker() using smaller parts (Suhas Daftuar)

Pull request description:

This is in preparation for re-using these validation components for a new version of AcceptToMemoryPool() that can operate on multiple transactions ("package relay").

Because of how our code handles SigChecks I could not remove the code commented under "Validate input
scripts against standard script flags" from the new PreChecks function into the PR's new PolicyScriptChecks
because since D5128 anything related to that number can only be run after CheckInputs.

The other code in the above new function was related to SegWit therefore I just removed the
PolicyScriptChecks function altogether.

Other missing members of the Workspace and ATMPArgs that were unused due to us not having RBF
were also taken out.

Backport of Core PR16400

Test Plan:

ninja all check check-functional

Reviewers: deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D8203