rqt_robot_dashboard) Remove doc files that contain no meaningful information (in favor of the default doc that sphinx generates once released).

Its current online rendition can be seen [here](http://docs.ros.org/hydro/api/rqt_robot_dashboard/html/doc/index.html), which is linked from its wiki. Assuming there's no one who will improve this document soon, it'd be far better to rely on auto-generated docs and link to it from wiki.