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.

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