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

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Backport of Core [[https://github.com/bitcoin/bitcoin/pull/17242 | PR17242]]

Test Plan:
  ninja all check check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

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

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