Remove isScalaJs/isScalaJvm on minor version update

[i10416]

Rel #384 (comment)
Now that scala.js(https://www.scala-js.org/news/2022/02/14/announcing-scalajs-1.9.0/) and scala native(scala-native/scala-native#2565) support java.util.BitSet, we no longer need these apis. Unfortunately these fields are public, so we will remove them in next minor version update to reduce downstream users' confusion.