remove remaining scala-java8-compat depedencies

[sebastian-alfers]

At some places we still use scala-java8-compat, as currently needed by com.github.matsluni.

Once 2.10.0 is out, try to bump https://github.com/matsluni/aws-spi-akka-http to get rid of the scala-java8-compat dependencies.

[jtjeferreira]

I faced a ClassNotFound exception when trying to upgrade a project using alpakka-kinesis to akka 2.10.x because aws-spi-akka-http still uses scala.compat.java8.OptionConverters. The workaround was to explicitly add a dependency on scala-java8-compat

[ennru]

Yes, you are right. It became obvious when I tried it in

• Drop unused compatibility libraries #3432