LoginPageLayout is overriding `is-paper` class from Vanilla on small screens

Login page layout is overriding `is-paper` class from Vanilla on small screens, hardcoding the white background there. It causes project that use this class, but also import react-component style, to have unintentional white background on small screens, where `is-paper` gray should be used.

This should be removed, or scoped to only apply to this component.

https://github.com/canonical/react-components/blob/a2ce1bcf3ace32bc0179f85a423441c32ef9538d/src/components/LoginPageLayout/LoginPageLayout.scss#L12-L14