Remove unused CDataStream methods

Summary:
This is a follow-up to D14689: change the callsites that use CDataStream::operator+= so that it can be removed.
This makes future backports related to streams.h easier.

This completes backport of core#20464
https://github.com/bitcoin/bitcoin/pull/20464/commits/faa96f841fe45bc49ebb6e07ac82a129fa9c40bf

Depends on D15903

Test Plan: ninja check check-seeder

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15905