Remove inconsistent subtests from color-valid-relative-color.html
These two cases expect relative colors to be resolved at parse time when it's possible to do so. However, the rest of the cases in this test have been updated per w3c/csswg-drafts#10328 to expect declared values of relative colors to preserve their original forms.
The cases tested that both rgb and color(srgb both result in color(srgb, but this is not valid any more for specified colors. We already have coverage of the same scenario for computed colors in color-computed-relative-color.html, so these subtests can be removed.