Remove dependency on web_tests/artifacts.

[eyebrowsoffire]

We no longer pass web_tests/artifacts via CAS, so we need to remove it as a dependency.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

[yjbanov]

[auto-submit]

auto label is removed for flutter/flutter/160700, due to - The status or check suite Windows build_tests_6_9 has failed. Please fix the issues identified (or deflake) before re-applying this label.