Drop deprecated mod_cluster subsystem resources/attributes and schemas since WF16 or earlier

Namely:

• proxy-list (string-based legacy configuration replaced by proxies=".." referencing outbound socket bindings; wasn't usable in runtime even due to a bug - https://issues.redhat.com/browse/WFLY-14130 - was only supporting mixed-domain)
• legacy proxy operations registered directly at subsystem level (that are registered at the proxy address /subsystem=modcluster/proxy=X:Y-operation()
• legacy ssl config (replaced by elytron)
• mem load metric (which was dropped from runtime for a long time; was never usable and was ignored in runtime for a long time)
• drop ZeroToNegativeOneParameterCorrector and fix attribute definitions for flush-wait, node-timeout, smax, ttl, and worker-timeout