Remove superfluous 'virtual' specifiers in ART.

Remove 'virtual' specifier on methods already bearing the 'override'
specifier.

Test: mmma art
Change-Id: I114930969a5ca048d88de9ecd18e2c6403593e31
diff --git a/dexlayout/dex_ir_builder.cc b/dexlayout/dex_ir_builder.cc
index 601d783..ca6ff9e 100644
--- a/dexlayout/dex_ir_builder.cc
+++ b/dexlayout/dex_ir_builder.cc
@@ -92,7 +92,7 @@
 template<class T> class CollectionMap : public CollectionBase {
  public:
   CollectionMap() = default;
-  virtual ~CollectionMap() override { }
+  ~CollectionMap() override { }
 
   template <class... Args>
   T* CreateAndAddItem(CollectionVector<T>& vector,