SMS messages incorrectly contain header and footer

As discussed on chat: https://chat.civicrm.org/civicrm/pl/oq68gypo5tdztkdzpuawnebk9y

An SMS sent via CiviCRM contained the plain text header and footer used for emails:

However, the opt-out link was broken. Including the header and footer also means the character count on the SMS compose form is wrong. Therefore it seems like SMSs should not actually include a header of footer.

I think this issue was introduced in https://github.com/civicrm/civicrm-core/commit/9b567ec2dbbb6a5dffe0805ca40098bfa454bd8a