[refactor] Remove unused ForEachNodeThen() template

Summary:
This is a backport of core#21186 [5/5]
https://github.com/bitcoin/bitcoin/pull/21186/commits/0829516d1f3868c1c2ba507feee718325d81e329

Depends on D10930

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10931