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.
Merge pull request #277 from gzm0/remove-isjsdefined

Fix #275: Remove isScalaJSDefined (and company)