Remove duplication from Whitehall.searchable_classes
Remove duplication from Whitehall.searchable_classes

Both SupportingPage and DocumentCollection are now edition classes,
so do not need to be present in additional_classes

YOTOV-LIMITED/whitehall,askl56/whitehall,ggoral/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,alphagov/whitehall,robinwhittleton/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,hotvulcan/whitehall,hotvulcan/whitehall,askl56/whitehall,robinwhittleton/whitehall,alphagov/whitehall,robinwhittleton/whitehall,askl56/whitehall,ggoral/whitehall,ggoral/whitehall,ggoral/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,alphagov/whitehall