RFR [XXS] : cleanup macosx jspawnhelper build settings

[Baesken, Matthias]

Hello,
    could I please have a  review for  the following small  change  that  cleans up  the special  macosx  BUILD_JSPAWNHELPER_DST_DIR setting that is not really
    needed any more after   CR 8066474: "Remove the lib/ directory from Linux and Solaris images"   changed the default setting .


Bug :

https://bugs.openjdk.java.net/browse/JDK-8173834

webrev :

http://cr.openjdk.java.net/~mbaesken/webrevs/8173834.0/


Thanks, Matthias