This change replaces our custom `panic` / `unwrap` lint with the one
from clippy. This rule as not properly applied in servoshell, so this
change fixes some clippy errors raised by the new configuration.
Testing: This change removes the tidy tests for the custom lints, but
otherwise the behavior is tested as part of clippy itself.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>