Adjust my prior change to better utilize structured form submission...

... to accomodate a plugins that are not using p:config-assignedLabel.

See: JENKINS-25372 for more information.
https://github.com/kohsuke/access-modifier
https://github.com/kohsuke/access-modifier/blob/master/access-modifier-annotation/src/main/java/org/kohsuke/accmod/restrictions/NoExternalUse.java#L8
https://github.com/kohsuke/access-modifier/blob/master/access-modifier-annotation/src/main/java/org/kohsuke/accmod/restrictions/DoNotUse.java#L32

Both prevent use in plugins despite being `public`, but the latter prevents use in code of the same module as well — which is probably a sensible default for these methods.