can't undefine task-max-threads

when undefining task-max-threads attribute, it will cause exception as follow.

`
[standalone@localhost:9990 /] /subsystem=io/worker=default:undefine-attribute(name=task-max-threads)
{
"outcome" => "failed",
"failure-description" => "WFLYCTL0158: Operation handler failed: java.lang.IllegalArgumentException",
"rolled-back" => true
}
`

although this fix can clear the model value, but the value in xnio is remainded,
can check it by read-attribute operation,
but reload is needed to modidy task-max-threads,
so IMHO it's not a issue.