refactor: Cleanup headers from walletmodel.h
Summary:
This is part 3 of 4 for backport of Core PR17513
Commit 5f50599
Depends on D7602
Test Plan: ninja && ninja check
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Subscribers: deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7603