This code is old and predates the improvements we made to the test
manifest to only contain working URLs (either Web Archive or
GitHub/Bugzilla links), so the fallback logic to try the Web Archive is
no longer necessary. This greatly simplifies the function and also
makes sure that we fail directly in case a bad URL is added to the
manifest, instead of having it work "accidentally" because of this
logic, since we want the manifest to be correct at all times (and
otherwise fail loudly).