tidy: Replace custom panic/unwrap lint with clippy lint (#38593)

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>

ports/servoshell/desktop/headless_window.rs

@@ -4,6 +4,9 @@
 
 //! A headless window implementation.
 
+#![deny(clippy::panic)]
+#![deny(clippy::unwrap_used)]
+
 use std::cell::Cell;
 use std::rc::Rc;