Remove unused _include_private argument

The implementation of respond_to? here is independent of the
implementation on Object which uses the include_private argument.

In this implementation the _include_private argument can and will never be used.

So remove it from the code.

lib/factory_bot/null_object.rb

@@ -13,7 +13,7 @@ module FactoryBot
       end
     end
 
-    def respond_to?(method, _include_private = false)
+    def respond_to?(method)
       @methods_to_respond_to.include? method.to_s
     end
   end