[css-values] attr() tests for rejecting `<url>` at parse time should be removed

[dshin-moz]

Test List

• attr-all-types.html: 1 subtest at the end here
• attr-security.html: 1 subtest here

Rationale

See discussion at w3c/csswg-drafts#13946.
Note that the discussion is around rejecting at parse time, and that they should still handled properly by attr tainting.