Remove old code for sending email to external users who create objects via inbound mail

Summary:
Ref T13493. I'm updating callers to getAccountID() to prepare to move it to a separate table.

This callsite once supported this flow:

• External users with no accounts send mail to bugs@.
• This creates tasks in Maniphest.
• They're CC'd when the tasks are updated.

However, after T12237 we never actually send this mail (since their addresses are necessarily unverified).

I left this code in in case this needed to be revisited, but it hasn't been an issue. Just remove it and treat these users as undeliverable.

Test Plan: As a cursory test for nothing being horribly broken, sent some object mail.

Maniphest Tasks: T13493

Differential Revision: https://secure.phabricator.com/D21007