Only undefine methods that were previously defined

In 241e8e5fb4, this call to `undef_method`
was introduced to suppress the warning printed by Ruby when a method is
redefined.

That warning is only printed when the method is already defined on the
class in question; it isn't printed when the method was inherited from
an ancestor, since overriding methods by shadowing them is a feature.

However, `method_defined?` returns true for inherited methods, which
means we're sometimes undefining methods that wouldn't cause a warning.

If a factory defines an attribute named `object_id` (admittedly not a
great idea), we will undefine the `object_id` method inherited from the
`Object` class, and the following warning will be printed:

    factory_bot/evaluator.rb:70: warning: undefining `object_id' may cause serious problems

By only undefining the method if it's defined on the current class, we
can avoid undefining inherited methods and triggering this warning.

lib/factory_bot/evaluator.rb

@@ -66,7 +66,7 @@ module FactoryBot
     end
 
     def self.define_attribute(name, &block)
-      if method_defined?(name) || private_method_defined?(name)
+      if instance_methods(false).include?(name) || private_instance_methods(false).include?(name)
         undef_method(name)
       end