Remove shadow DOM from `atom-text-editor`

This _is_ a really great pull request, but I thought it was submitted because it's a great example of overusing shiny new web features, then having to roll back that shit because it didn't work out and the boring solution was the better one.

I submitted this because it's interesting to me in the context of the web platform and Atom, and because it's a great example of how to write a really clear pull request.