[backport#17677] Activate watchonly wallet behavior for LegacySPKM only

Summary:
e1e1442f3eadc1d139380e71c1b60b86d8d6bdee Activate no-privkey -> ISMINE_WATCH_ONLY behavior for LegacySPKM only (Gregory Sanders)

Pull request description:

Slight cleanup following https://github.com/bitcoin/bitcoin/pull/16944

This should allow future scriptpubkeymans to transparently work, since the current plan is to have ismine always be spendable.

Backport of Core PR17677

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7728