Remove reply handler instructions from email

Summary:
Ref T7199. Although this is useful for discovery, it's un-useful enough that we already have an option to disable it, and most applications do not provide any meaningful instructions.

Throwing it away makes it easier to move forward and lets us get rid of a config option.

This is becoming a more advanced/power-user feature anyway, and the new syntax will be significantly more complex and hard to explain with a one-liner. I'm currently thinking that I'll maybe make the "help" menu a dropdown and give it some options like:

+---+
| O |
+---+---------------------+
| Maniphest Documentation |
| Maniphest Email Actions |
+-------------------------+

Then you click the "Email Actions" thing and get a runtime-derived list of available options. Not sure if I'll actually build that, but I think we can fairly throw the in-mail instructions away even if we don't go in that specific direction.

Test Plan: Grepped for replyHandlerInstructions, got no hits.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12229