Remove experimental DHCPv6 prefix-class support.

The standard for this never made it beyond an internet-draft which expired
in 2012, so it can be considered dead, I think.

src/dhcp6-protocol.h

@@ -61,10 +61,6 @@
 #define OPTION6_FQDN            39
 #define OPTION6_CLIENT_MAC      79
 
-/* replace this with the real number when allocated.
-   defining this also enables the relevant code. */ 
-/* #define OPTION6_PREFIX_CLASS    99 */
-
 
 #define DHCP6SUCCESS     0
 #define DHCP6UNSPEC      1