[WFLY-3675] Remove mod_cluster ContextHost and Proxy classes and use res...

...olveModelAttribute() instead

https://issues.jboss.org/browse/WFLY-3675

Goodbye hacky classes.
Deleting is fine, but please be sure this id isn't reused. There's a mapping file in the EAP source tree from the old message ids to the new, and that mapping shouldn't end up pointing to some unrelated message.