Remove the Sundown submodule from the repository

Hello,

Since Sundown is deprecated we can remove it from the submodule list and we won't pull code from it anymore so we can remove related rake tasks. This way, the `tasks` folder is useless so let's gather tests tasks and Rakefile.

I would love to have your thoughts @mattr- and @vmg. :smiley: 

Have a nice day.