Hide auto-sanitize option when Native theme support is built-in

[westonruter]

When theme support is not built-in and the user selects the Native template mode, the auto-sanitize checkbox is hidden since it is forced. However, this isn't currently working when theme support is built-in via add_theme_support('amp'):

[westonruter]

The problem lies here:

https://github.com/Automattic/amp-wp/blob/0401d5e971aa29c3fc2c9981af41ca51e7d807cf/includes/options/class-amp-options-menu.php#L261-L262

When AMP support is built-in the jQuery( 'input[type=radio][name="amp-options[theme_support]"]' ) result is empty because there are no template mode radio buttons.

[kienstra]

Moving To 'Ready For Merging'

If it's alright, I'm moving this to 'Ready For Merging,' as I don't think it needs functional testing. Feel free to move it back.