[scroll-animations] Remove @scroll-timeline-specific tests

These tests are specific to how @scroll-timeline worked, and can't
be converted to the new API.

The only potential exception is the "boxless" test in
at-scroll-timeline-source.html. I've filed Issue 1356546 so we don't
forget to add a WPT for that later.

Bug: 1317765, 1356546
Change-Id: I60ca512691bb3e43a787eea353cc655b9cc015df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3856742


Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040599}