[backport#17304 14/18] Refactor: Move HavePrivateKeys code out of CWallet::CreateWalletFromFile

Summary:
This commit does not change behavior.

https://github.com/bitcoin/bitcoin/pull/17304/commits/f45d12b36cee05aa3c2685b951d27bd8a58539ae

Depends on D7160

Partial backport of Core PR17304

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7161