remove old target: it is on the classpath and may break scalac;

--- a/src/Pure/build-jars	Mon Oct 24 14:47:46 2016 +0200
+++ b/src/Pure/build-jars	Mon Oct 24 15:00:13 2016 +0200
@@ -242,6 +242,7 @@
     done
   }
 
+  rm -f "$TARGET"
   rm -rf classes && mkdir classes
 
   SCALAC_OPTIONS="$ISABELLE_SCALA_BUILD_OPTIONS -d classes"