SI-8764 remove Xexperimental special Product members for case classes

`productElement` and `productIterator` return types under Xexperimental
use the weak lub of the case class's parameters instead of just Any.
This isn't really useful, nor tested and it is broken (SI-8764) anyway.

The special casing of `productElement` and `productIterator` was
introduced with `ProductN` support in case classes (e412524), but was
not removed when `ProductN` support went away (70996f8, d7f498a,
b7395e9, 317a105).
Merge pull request #3970 from gourlaysama/wip/t8764-product-case-class

SI-8764 remove Xexperimental special Product members for case classes