Revert "Restore current_board_json method"

This reverts commit 3c7eebdd.

ee/app/helpers/ee/boards_helper.rb

@@ -62,16 +62,5 @@ module EE
         s_("IssueBoards|Board")
       end
     end
-
-    def current_board_json
-      board = @board || @boards.first
-
-      board.to_json(
-        only: [:id, :name, :milestone_id],
-        include: {
-          milestone: { only: [:title] }
-        }
-      )
-    end
   end
 end