R1:a0668df75a2d

Remove "metamta.domain" and "metamta.placeholder-to-recipient" config options

Summary:
Ref T920. This simplifies mail configuration.

The "metamta.domain" option is only used to generate Thread-ID values, and we just need something that looks like a bit like a domain in order to make GMail happy. Just use the install domain. In most cases, this is almost certainly the configured value anyway. In some cases, this may cause a one-time threading break for existing threads; I'll call this out in the changelog.