[w3c/editing] remove document.designMode (#177)

This is just an idea from [#176](https://github.com/w3c/editing/issues/176#issuecomment-400949155).

`document.designMode = true` is same as `document.documentElement.contenteditable = true` and it looks there is rarely any usecase today.
Since I don't have any metrics so far, how about removing this feature for simpleness?


-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/editing/issues/177