test: Remove or explain syncwithvalidationinterfacequeue

Summary:
syncwithvalidationinterfacequeue is a hidden test-only RPC, so it should not be used when it is not needed. Thus, either remove it or explain why it is needed.

Revert "test: Add missing sync_all to wallet_balance test"

This reverts commit rABC7a820a75ecfb

The underlying bug has been fixed in D7511.

test: Document why syncwithvalidationinterfacequeue is needed in tests

This is a backport of core#19893

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10895