Drop DelayedInit

[adriaanm]

Deprecation: scala/bug#5479

Special-case App trait instead?

[dwijnand]

I think the twin Dotty ticket here is scala/scala3#559.

[SethTisue]

thread in DelayedInit's defense: https://contributors.scala-lang.org/t/scala-3-delayedinit/1902

[SethTisue]

I think this is out of scope for Scala 2.