Remove unused helper method Spree::Admin::BaseHelper#back_to_list_button
Remove unused helper method Spree::Admin::BaseHelper#back_to_list_button

We don’t use those back to list buttons anymore as we have the
breadcrumb navigation in place.

calvinl/spree,ayb/spree,DarkoP/spree,vinsol/spree,vinayvinsol/spree,siddharth28/spree,DarkoP/spree,siddharth28/spree,vinsol/spree,imella/spree,calvinl/spree,vinayvinsol/spree,brchristian/spree,vinsol/spree,imella/spree,vinayvinsol/spree,brchristian/spree,brchristian/spree,ayb/spree,ayb/spree,DarkoP/spree,DarkoP/spree,vinayvinsol/spree,siddharth28/spree,ayb/spree,calvinl/spree,siddharth28/spree,brchristian/spree,imella/spree,calvinl/spree,vinsol/spree