Remove "Add panel" item from panel context menu

Having an "Add Panel" item in the panel context menu communicates to the
user that the new panel will be placed on the existing panel somehow, or
at least will bear some relationship to it. However this is false; the
new panel is added to the desktop containment, not the panel
containment. Therefore it is inappropriate to have this item in the
panel context menu since it has nothing to do with the panel that has
been right-clicked on.