Deprecate and schedule for removal redundant mod_cluster add/remove-[custom-]metric operations

There is a bunch of nonsensical operations registered at the mod_cluster configuration resource, see https://wildscribe.github.io/WildFly/12.0/subsystem/modcluster/mod-cluster-config/configuration/index.html

These are:

• add-custom-metric Add new custom metric to the load balancer provider.
• add-metric Add new metric to the load balancer provider.
• remove-custom-metric Remove a custom metric from the load balancer provider.
• remove-metric Remove a metric from the load balancer provider.

These should have been deprecated long time ago, since they are just redundant. Regular model :add/:remove operations should be used for manipulating load metrics.