Required radio buttons in Likert elements have invalid aria-required markup

Removes the aria-required attribute from the Likert PHP and related test, where it checks for the final markup including aria-required. I think this has all the relevant Likert files from a quick search, but if there are other files I'm missing, that'd be good to know.

There are other webform helper modules where aria-required does appear (Webform Rating, Webform Example, etc.), but not sure if we want to broaden this ticket to include fixes for those files too.

Automatically closed - issue fixed for 2 weeks with no activity.