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 also reverts the other change in fb6e687,
which introduced a case for RefinedType, which are
not needed to pass pos/t6556.scala.

Reviewed by @paulp in it's previous incarnation, #1868