remove redundant configuration

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1646700 13f79535-47bb-0310-9956-ffa450edef68
Verify OS version instead of Java version.
When specifying invoker.javaHome, the Java version of the Maven runtime can be different then the script runtime (which still uses the original runtime aka JAVA_HOME). So use OS, which should be stable.

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1646725 13f79535-47bb-0310-9956-ffa450edef68