simplify scala.runtime.Statics.releaseFence

[xuwei-k]

https://www.scala-lang.org/news/next-scala-lts-jdk.html

JDK 17 will be the new minimum version.

• Offer stronger guarantees under the Java Memory Model for List & Vector (using releaseFence) scala#6425

The runtime support is a bit complicated becausse we need to be cross
compatible with JDK 8 and 9+. We use method handle based reflection
to achieve this. This has zero runtime overhead in the manner used.

I think it's time to delete JDK 8 compatible reflection code.