Do not encode a contact message twice

Updated: Comment #0

Problem/Motivation

Follow-up from #1856562-31: Convert "Subject" and "Message" into Message base fields
MessageViewBuilder does check_plain() for message body and calls drupal_html_to_text() twice to revert body HTML entities, so mail system does not get a original message.
If a some contrib module will try to swap mail backend there's no way to get original message body.

Proposed resolution

Remove magic conversions, because core's mail already cares to filter mail body to plain text.

Remaining tasks

User interface changes

API changes