Remove classloader `null` workaround

[ssorallen]

As of Play 2.3.6, it more accurately chooses its class loader which should prevent the null workaround when trying to find the Nashorn JS engine. Test if that hypothesis is true and remove the null argument if it is no longer needed.

The class loader fix: playframework/playframework#3420

[ssorallen]

Actually that pull request was merged into master but not into the 2.3.x branch. The fix will go into Play 2.4.x, which means this fix is on hold until then.