fix(@clayui/css): Exclude variant styles from being output when using c-unset

[pat270]

fixes #5471

We can prevent variant styles from being output with this pattern:

$alert-palette: (
    primary: c-unset,
);

$btn-sizes: (
    btn-xs: c-unset,
);

$btn-monospaced-sizes: (
    btn-xs: c-unset,
);

previously it would throw an error.

[matuzalemsteles]

LGTM