Drop ssh-async rex mode

[adamruzicka]

Companion to theforeman/smart_proxy_remote_execution_ssh#128

[adamruzicka]

If ssh-async mode was previously configured, the failure will look like this

2025-11-26 11:05:27 [ERROR ] [root] Parameter foreman-proxy-plugin-remote-execution-script-mode invalid: ssh-async must be one of ssh, pull-mqtt

Having it fail more gracefully would probably mean continue listing ssh-async as valid option, but then catching it at run time and failing hard, which I'd like to avoid.