Delete `requires_input.js` behavior

What does this MR do and why?

Delete requires_input.js behavior

Remove this behavior which disables submit button when required fields are empty, as this is not recommended UX

Changelog: changed

MR acceptance checklist

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Screenshots or screen recordings

Submitting an empty form will now show a HTML5 validation error message

How to set up and validate locally

1. Test a form formerly with js-requires-input, e.g. http://127.0.0.1:3000/-/user_settings/gpg_keys