Remove PHP Markdown
Squashed 'includes/libraries/parsedown/' content from commit 93f7b26

git-subtree-dir: includes/libraries/parsedown
git-subtree-split: 93f7b26427737f8325c81ce111b33011aa9013f9
Merge commit 'b330d2259dbc2b8994c25db325411b740e53730a' as 'includes/libraries/parsedown'