remove 'mode' property when creating a new CodeMirror control. This appears to…

remove 'mode' property when creating a new CodeMirror control. This appears to completely crash if the user puts in a <style ...> tag in the input. This was causing imported static tabs to not be editable

common/lib/xmodule/xmodule/js/src/html/edit.coffee

 class @HTMLEditingDescriptor
   constructor: (@element) ->
     @edit_box = CodeMirror.fromTextArea($(".edit-box", @element)[0], {
-      mode: "text/html"
       lineNumbers: true
       lineWrapping: true
     })