[net] Delete CNetMessage copy constructor/assignment op

Summary:
This is a partial backport of core#27324
https://github.com/bitcoin/bitcoin/pull/27324/commits/b5a85b365a4abd98176b0935015dbb502cc3e6f6
Depends on D18247

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18248