Remove scala-continuations-plugin from scala-library-all

The continuations plugin should instead be a dependency of scala-dist,
as scala-library-all should be a drop-in replacement for scala-library,
and as such should not (indirectly) depend on plugins/the compiler.
Merge pull request #3661 from adriaanm/plugin-not-in-scala-library

Remove scala-continuations-plugin from scala-library-all