🐛Clear the hidden input elements on the clear action.

[cvializ]

Fixes #14308

amp-selector creates hidden inputs so that it behaves like a radiobutton or checkbox group in a form. When selecting normally these are created and destroyed as the user selects and unselects options. Using the clear AMP action to unselect all the options would not remove these hidden inputs and the selector would incorrectly submit out-of-sync data in a form. This PR removes the hidden inputs on the clear AMP action and tests that no hidden inputs remain afterward.

[cathyxz]

This looks good. So apparently the setInputs_ function is used to set the selection each time a user selects an option. It removes all the options one by one and recreates them. That seems to be a bit of overkill. git blames to #5975, but unfortunately I can't decipher why it was implemented this way. Maybe it's possible to make this more efficient.

[cvializ]

Thanks for the review! I opened a Discussion/Question issue to track possible improvements for the hidden inputs. I'll merge this now to fix the behavior.