Remove .editor class to force themes to style background via :host

The goal is to allow the editor’s styling to be overridden as follows:

atom-text-editor {
  background: black;
  color: white;
}

If we retain these classes, themes will continue to style the root
element of the editor and destroy the ability to style the editor in
this way. You would instead have to do the following:

atom-text-editor::shadow .editor {
  background: black;
  color: white;
}

This is way less intuitive and confusing for people.

src/text-editor-component.coffee

@@ -90,7 +90,10 @@ TextEditorComponent = React.createClass
 
       style.height = scrollViewHeight if @autoHeight
 
-    className = 'editor-contents'
+    if useShadowDOM
+      className = 'editor-contents--private'
+    else
+      className = 'editor-contents'
     className += ' is-focused' if focused
     className += ' has-selection' if hasSelection