Revert diagnostics code from 1251537459

components/style/custom_properties.rs

@@ -168,13 +168,11 @@ where
             return;
         }
         self.index.retain(|key| !set.contains(key));
-        self.values.begin_mutation();
         // XXX It may be better to use retain when we back to use a
         // normal hashmap rather than DiagnosticHashMap.
         for key in set.iter() {
             self.values.remove(key);
         }
-        self.values.end_mutation();
         debug_assert_eq!(self.values.len(), self.index.len());
     }
 }