refactor: Drop no longer used swap member functions
Summary:
This is a partial backport of core#26749
https://github.com/bitcoin/bitcoin/pull/26749/commits/cea50521fe810111a8a3c84ad14f944eafb5b658
Depends on D16145
Test Plan: ninja && ninja check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D16146