Remove loading=lazy attribute from iframes

[pierlon]

Summary

This PR removes the loading="lazy" attribute when sanitizing iframes as amp-iframe already has lazy loading built-in. All other values for the loading attribute are to result in validation errors.

Fixes #3939.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).