Clear editor holder

This is for #2379. EditorManager._editorHolder is set to a temporary DOM object in createMockEditor(), but it is not cleared in destroyMockEditor() after temporary DOM object is destroyed, and EditorManager holds on to this object (because Brackets only ever has a single, permanent _editorHolder) that it uses for all resizing events.

EditorManager._editorHolder is now cleared before actual DOM object is destroyed.

@peterflynn : I think you also looked at this, so care to review it?