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.
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