test: remove unused code from p2p_compactblocks

Summary:
This code is unused since D1172 (we used to check that the tx didn't have a witness)

Also remove comments reffering to code that was removed in D1611 and D3814

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18711