Remove explicit lifetime arguments on a method call.

This causes a warning it today’s Nightly:
rust-lang/rust#42868

… which makes the build fail because we use `#![deny(warnings)]`.
This warning is planned to become a hard error in a future Rust version.

components/style/stylesheets/stylesheet.rs

@@ -240,7 +240,7 @@ pub trait StylesheetInDocument {
         device: &'a Device,
         guard: &'a SharedRwLockReadGuard<'b>
     ) -> EffectiveRulesIterator<'a, 'b> {
-        self.iter_rules::<'a, 'b, EffectiveRules>(device, guard)
+        self.iter_rules::<EffectiveRules>(device, guard)
     }
 
     rule_filter! {