Use ATTRIBUTE_UNUSED more.

Use it in lieu of UNUSED(), which had some incorrect uses.

Change-Id: If247dce58b72056f6eea84968e7196f0b5bef4da
diff --git a/runtime/gc/accounting/remembered_set.cc b/runtime/gc/accounting/remembered_set.cc
index 277d319..eb0852a 100644
--- a/runtime/gc/accounting/remembered_set.cc
+++ b/runtime/gc/accounting/remembered_set.cc
@@ -41,8 +41,7 @@
   explicit RememberedSetCardVisitor(RememberedSet::CardSet* const dirty_cards)
       : dirty_cards_(dirty_cards) {}
 
-  void operator()(uint8_t* card, uint8_t expected_value, uint8_t new_value) const {
-    UNUSED(new_value);
+  void operator()(uint8_t* card, uint8_t expected_value, uint8_t new_value ATTRIBUTE_UNUSED) const {
     if (expected_value == CardTable::kCardDirty) {
       dirty_cards_->insert(card);
     }