net: remove misplaced and outdated comment

Summary:
The comment was previously attached to m_addr_known, but now it is attached to id, which is wrong.

This comment would have been moved to the correct place in core#20653, and then moved again and rephrased in core#21186 (we added that comment in D10929)

Depends on D18244

Test Plan: trivial

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18245