!str #16039 Remove old scaladsl, rename scaladsl2

* and impl2, testkit2
* keeping io2 for now

Are there any more things in impl2 that are not used?

One failing test (ignored for now): io2.TcpFlowSpec
See FIXME in that test. What can I have done?
I have no opinion. I can revert that, and change my IDE settings.