[css-color-adjust] Remove an invalid test.

This test was added in:

  https://chromium-review.googlesource.com/c/chromium/src/+/2230909

However I don't understand why it should pass. There's nothing that
guarantees that `(prefers-color-scheme: dark)` matches on the SVG, and
the test is consequently failing in all browsers:

  https://wpt.fyi/results/css/css-color-adjust/rendering/dark-color-scheme/svg-as-image.html?label=master&label=experimental&aligned

I guess it could be nice if `color-scheme` affected
`prefers-color-scheme` for svg-as-image, maybe, but afaict that was not
what the test was about, nor something that is specified.