Remove Markdown support

Follow-up to #2643882: Add Markdown support

@Wim Leers pointed out that Markdown and CKeditor don't mix well, which could introduce complexities if we decide to use D8's built-in WYSIWYG in the future. See conversation starting here: https://www.drupal.org/node/2361735#comment-10790670

It's possible for farmOS users to add Markdown to their own individual sites if they want, so it doesn't make sense to include in by default in farmOS if that means D8 upgrade issues.

Flip-flopping ain't always a bad thing. ;-)