Remove the NONE case from the OS enum.

The `none` os constraint value remains as a standard platform.

While names such as 'freestanding' or 'standalone' have been proposed,
the name remains 'none' to be consistent with the naming of targets
triples such as 'arm-none-eabi' that will satisfy this constaint.

src/main/java/com/google/devtools/build/lib/util/OS.java

@@ -23,7 +23,6 @@ public enum OS {
   FREEBSD("freebsd", "FreeBSD"),
   LINUX("linux", "Linux"),
   WINDOWS("windows", "Windows"),
-  NONE("none", "No OS"),
   UNKNOWN("unknown", "");
 
   private static final EnumSet<OS> POSIX_COMPATIBLE = EnumSet.of(DARWIN, FREEBSD, LINUX);