Remove unused `before_filters`

This method added by 10085114. It is unnecessary because it is no longer called
by 19c3495a.

actionpack/lib/action_controller/metal/testing.rb

@@ -12,11 +12,5 @@ def recycle!
         self.params = nil
       end
     end
-
-    module ClassMethods
-      def before_filters
-        _process_action_callbacks.find_all { |x| x.kind == :before }.map(&:name)
-      end
-    end
   end
 end