Remove public/400.html error page
Remove public/400.html error page

This was incorrectly committed in fe53e32.

telekomatrix/e-petitions,joelanman/e-petitions,unboxed/e-petitions,unboxed/e-petitions,unboxed/e-petitions,alphagov/e-petitions,oskarpearson/e-petitions,StatesOfJersey/e-petitions,alphagov/e-petitions,oskarpearson/e-petitions,joelanman/e-petitions,StatesOfJersey/e-petitions,ansonK/e-petitions,ansonK/e-petitions,oskarpearson/e-petitions,alphagov/e-petitions,alphagov/e-petitions,telekomatrix/e-petitions,unboxed/e-petitions,StatesOfJersey/e-petitions,joelanman/e-petitions,StatesOfJersey/e-petitions,telekomatrix/e-petitions,ansonK/e-petitions