RFR 9: 7021006: (process) Remove disabled clone-exec feature

[Roger Riggs]

At one time clone() was used on Unix to spawn processes but no more.
Please review the cleanup of now dead code.

Webrev:
    http://cr.openjdk.java.net/~rriggs//webrev-clone-7021006

Issue:
   https://bugs.openjdk.java.net/browse/JDK-7021006

Thanks, Roger