Do not enter Scala 2 extension methods and let dotty "generate" them

[hamzaremmal]

Scala 2 generates extension methods for Value Classes before pickling while Scala 3 generates them after.
Now that we are working on compiling the Scala 2 stdlib with dotty (#22480), and therefore generating tasty files for the stdlib, we will have to distinguish between scala 2 that was unpickled and scala 2 code coming from tasty. For that, we currently have a flag, but it is not very useful apart from some very specific details where a workaround exists. This PR, removes one of 2 places where this flag is used. The second place will be removed once we inline the missing symbol in Predef.scala (after #22480) is merged.

[hamzaremmal]

I've minimized the issue in the community build B to the following:

trait Bar[T]
class Foo[T <: Bar[T]] (private val buffer: Any) extends AnyVal

I'm a bit surprised because it doesn't use any Scala 2 classes and we don't need to have the -Ycompile-scala2-library

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EDIT:With -Xprint:all, the compiler doesn't crash

[hamzaremmal]

Blocked by #22548