R1:29923cc71a46

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…