Body HTML should not be a required field in automated messages

In automated messages such as subscription confirmations, Body HTML is a required field.

This is inconsistent with the rest of CiviMail, in which including an HTML part is optional.

Conceptually, I can't think of any reason why an HTML part would be a requirement here.

While we do include HTML parts in other messages, we want to remove that part from confirmation messages, because we want to absolutely minimize their chance of being flagged as spam. Because confirmation messages are often shorter, with a low ratio of words to link, they are more likely to be flagged. Including only a plain text part would help lower their score.