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/runtime/jni/java_vm_ext_test.cc b/runtime/jni/java_vm_ext_test.cc
index fe1c168..4049c6e 100644
--- a/runtime/jni/java_vm_ext_test.cc
+++ b/runtime/jni/java_vm_ext_test.cc
@@ -34,7 +34,7 @@
   }
 
 
-  virtual void TearDown() override {
+  void TearDown() override {
     CommonRuntimeTest::TearDown();
   }