[backport#17304 8/18] refactor: Replace UnsetWalletFlagWithDB with UnsetBlankWalletFlag in ScriptPubKeyMan

Summary:
ScriptPubKeyMan is only using UnsetWalletFlagWithDB to unset the blank
wallet flag. Just make that it's own function and not expose the flag
writing directly.

This does not change behavior.

https://github.com/bitcoin/bitcoin/pull/17304/commits/fc2867fdf50f47e5ad5f43445e500e3a477a8074

Depends on D7154

Partial backport of Core PR17304

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7155