SI-8764 remove Xexperimental special Product members for case classes

See #3938 for the original PR targeted to 2.11.x.

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`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).