Revert notes feature from 3.0 release

[ocean90]

The current notes/comments implementation was pushed via #209. Unfortunately, the merged PR #975 isn't yet ready for a public release. Bugs already have been reported in #998, #999, or #1000. I also did another quick test:

• I was able to submit an empty note.
• After I submitted a note the next translation opened.
• The note wasn't visible after a reload.
• The layout is broken

The other remaining issue is that there are no (global) tools to support moderating the discussions.

That's why I'm proposing to remove the current implementation. This should give us the chance for another try with a clean state in a later version.
This was also discussed in person at WCEU with the same conclusion.