Don't store IP addresses in content sources

Summary:
We don't use these for anything, we're inconsistent about recording them, and there's some mild interaction with privacy concerns and data retention. Every other log we store any kind of information in can be given a custom retention policy after recent GC changes.

If we did put this back eventually it would probably be better to store a session identifier anyway, since that's more granular and more detailed.

You can fetch this info out of access logs anyway, too.

Test Plan: Left a couple of comments.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14315