SI-6556 Remove unneeded workaround in erasure.

In fb6e687, a workaround was added, described as:

> See SI-6556. It seems in some cases the result constructor
> type of an anonymous class is a different version of the class.
> This has nothing to do with value classes per se.
> We simply used a less discriminating transform before, that
> did not look at the cases in detail.
> It seems there is a deeper problem here, which needs
> following up to. But we will not risk regressions
> in 2.10 because of it.

I was expecting an arduous investigation into this,
but happilly the underlying problem was already solved
in the fix for SI-6648 (1587a77e). (Chalk up another
win for libscala's scala-hash!)

This commit escalates the logging to an assertion.
It _doesn't_ revert the other change in fb6e687,
which introduced a case for RefinedType. Those lines
are not needed to pass pos/t6556.scala, but I'll
take the decision to remove those lines under advisement
of a reviewer.

Review by @odersky

  - Is this case now redundant? If not, what test case could we add to exercise it? https://github.com/retronym/scala/blob/cced84a3/src/reflect/scala/reflect/internal/transform/Erasure.scala#L217

And by @adriaanm for a call on `2.10.x` vs `master`.