[backport#17242] refactor: Remove unused cacheSigStore from CheckInputsFromMempooAndCache

Summary:
0a433fc876d82df1005f175c1254fff62f0f36f8 [validation] Remove unused cacheSigStore from CheckInputsFromMempoolAndCache (John Newbery)

Pull request description:

CheckInputsFromMempoolAndCache() is only called in one place, and
cacheSigStore is set to true in that call site. Remove the argument
entirely.

Also improve commenting.

Backport of Core PR17242

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8753