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_test.cc b/dex2oat/dex2oat_test.cc
index 5fc3da4..b2fe441 100644
--- a/dex2oat/dex2oat_test.cc
+++ b/dex2oat/dex2oat_test.cc
@@ -55,7 +55,7 @@
 
 class Dex2oatTest : public Dex2oatEnvironmentTest {
  public:
-  virtual void TearDown() override {
+  void TearDown() override {
     Dex2oatEnvironmentTest::TearDown();
 
     output_ = "";