Fix #5827: Don't copy protected flag to apply method of case class

[odersky]

No description provided.

[Jasper-M]

On the one hand, with the current semantics of protected a protected method in an object is a bit silly as it is more or less the same as a private one. But on the other hand, it's a bit inconsistent to special case private. And why would anyone make a constructor protected if they still automatically get a public factory method?

Actually I would half expect the companion object of a class to have access to the protected members of the superclass of that class, and of the companion object of that superclass. But that's another can of worms...

[odersky]

@Jasper-M protected simply makes no sense in an object, so we have to drop it.