[protocol] Remove unused CADDR_TIME_VERSION

Summary:

These constants are no longer required and can be removed.

Additional code comments are added to explain CAddress serialization.

Backport of core PR19486.

Depends on D8514.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8515