refactor: Drop ::HasWallets()

Summary:

Minor follow-up of core#19250. The global HasWallets() is used only once and at the call site there's already a way to know if any wallet is loaded.

This is a backport of core#19261

Depends on D9456

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9457