Fix #275: Remove isScalaJSDefined (and company)

The Scala.js compiler relies on the ScalaJS library being on the classpath since the introduction of RuntimeLong. Therefore, checking for `isScalaJSDefined` can only achieve the following two:

- Increase clutter
- Give the wrong impression that the ScalaJS library need not be on the classpath.

This commit removes `isScalaJSDefined` and references to it.