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/dex2oat/dex2oat_image_test.cc b/dex2oat/dex2oat_image_test.cc
index 60dba99..e047b4f 100644
--- a/dex2oat/dex2oat_image_test.cc
+++ b/dex2oat/dex2oat_image_test.cc
@@ -52,7 +52,7 @@
 
 class Dex2oatImageTest : public CommonRuntimeTest {
  public:
-  virtual void TearDown() override {}
+  void TearDown() override {}
 
  protected:
   // Visitors take method and type references