RFR: JDK-8173822 Remove dead code in BuildNashorn.gmk

[Magnus Ihse Bursie]

There are dead code inBuildNashorn.gmk that is related to the now 
defunct nashorn.jar. It should be removed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8173822
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8173822-remove-nashorn-jar-cleanup/webrev.01

/Magnus