removed class ipv3_option_impl and class ipv6_option_impl (message_impl.hpp) (#515)

* modified ipv3_option_impl to ipv4_option_impl

* removed class ipv3_option_impl  and class ipv6_option_impl, because it is not necessary

Author: ziyangfu

implementation/service_discovery/include/message_impl.hpp

@@ -36,8 +36,6 @@ class serviceentry_impl;
 
 class option_impl;
 class configuration_option_impl;
-class ipv3_option_impl;
-class ipv6_option_impl;
 class load_balancing_option_impl;
 class protection_option_impl;
 class selective_option_impl;