Remove unused test (#20117)

This PR cleans up some old stale test that has been skipped from the
beginning.

While this test wants to prove that migrating from Tailwind CSS v3 to
Tailwind CSS v4 can handle the `#{!important}` SCSS notation, it doesn't
prove that we can migrate an entire SCSS project. SCSS has much more
special syntax and we never supported that.

Let's get rid of this test that's doing nothing at the moment.
Especially since we don't want to migrate SCSS projects. Enabling this
might result in the false sense that we _do_ support SCSS migrations
which is not the case.

Closes: #20106