Remove documentation about obsolete Differential event listeners

Summary:
Fixes T5069. T2222 mostly-intentionally stopped emitting these.

My sense is that users generally find event listeners (or, really, writing PHP at all) much less preferable to things like Herald rules or HTTP hooks. This is generally good, since those things are way easier to maintain, so I plan to continue moving away from events in cases where we have reasonable alternatives.

We also generally have more and better alternatives now than when these were written.

Test Plan: grep

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5069

Differential Revision: https://secure.phabricator.com/D9151