Fix message preview template

HTML preview of message in admin is never shown no matter whether I have
`templates/newsletter/message/message.html` file in my project or not.

In `newsletter/templates/admin/newsletter/message/preview.html` template showing of HTML preview depends on if statement which uses attribute
`message.newsletter.message_template.html` that don't exist anymore after moving to file based message templates.

Fixed by removing that if statement.

HTML version of message is supposed to be optional. With database
templates it depended on existence of HTML template but with current
file based templates default HTML template for message always exist. So
other mechanism is needed.