Fix some build warnings

- Mark some instances of unused fields and variables as as allowed,
  when they are used for memory management.
- Remove the use of some deprecated function.s

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/bindings/principals.rs

@@ -115,6 +115,7 @@ impl Deref for ServoJSPrincipalsRef<'_> {
     }
 }
 
+#[allow(unused)]
 pub unsafe extern "C" fn destroy_servo_jsprincipal(principals: *mut JSPrincipals) {
     Box::from_raw(GetRustJSPrincipalsPrivate(principals) as *mut MutableOrigin);
     DestroyRustJSPrincipals(principals);