Remove "feed.publish" API

Summary:
Ref T13681. This was introduced in D593, never used, and doesn't make sense or have any uses in modern Phabricator.

It also does some pretty direct writes that can technically do things that at least look like they violate policies, so remove it.

Test Plan:

• Checked the API console, no longer saw "feed.publish".
• Grepped for "feed.publish", no hits.

Maniphest Tasks: T13681

Differential Revision: https://secure.phabricator.com/D21826