Convert previously implemented editor-only elements to using the newly added editor-only class

With the addition of the class `WYMeditor.EDITOR_ONLY_CLASS` in issue #414 that can be used to mark an element as editor-only so that it is removed by the XHTML parser, there is no longer a need to use the previous methods for removing elements from the editor that should not be included with the XHTML output. These previous methods should now be replaced by using the `WYMeditor.EDITOR_ONLY_CLASS` on elements that need to be editor-only.