R1:9d5b8bd14a97

Remove PHPMailer code which generates bogus "Message-ID" email headers

Summary:
See <https://discourse.phabricator-community.org/t/how-to-override-localhost-localdomain-in-email-message-id/3876/>.

Currently, Phabricator generates a "Message-ID" only in a subset of cases (roughly: when the message is first-in-thread and we expect the thread may have more than one message).

In cases where it does not generate a message ID, it expects the SMTP server to generate one for it. Servers will generally do this…