Add legacy ‘from-top’ class at the view layer, not the model

This blends in to our approach with the other legacy panel classes.

src/workspace.coffee

@@ -682,8 +682,6 @@ class Workspace extends Model
   #
   # Returns a {Panel}
   addModalPanel: (options={}) ->
-    # TODO: remove these default classes. They are to supoprt existing themes.
-    options.className ?= 'overlay from-top'
     @addPanel('modal', options)
 
   addPanel: (location, options) ->