Convert the remaining MOJO_OVERRIDE -> override, and get rid of MOJO_OVERRIDE.

mojo/public/cpp/system/macros.h

Index: mojo/public/cpp/system/macros.h
diff --git a/mojo/public/cpp/system/macros.h b/mojo/public/cpp/system/macros.h
index a6551480481f9c88daf197a7d963c0236ff1a0c4..605906e4a335be210705cae6440ae0ee2c3561ed 100644
--- a/mojo/public/cpp/system/macros.h
+++ b/mojo/public/cpp/system/macros.h
@@ -11,12 +11,6 @@
 // Mojo C++ API users can assume that mojo/public/cpp/system/macros.h
 // includes mojo/public/c/system/macros.h.
 
-// TODO(vtl): Get rid of this once all users have been eliminated.
-// Annotate a virtual method indicating it must be overriding a virtual method
-// in the parent class. Use like:
-//   virtual void foo() OVERRIDE;
-#define MOJO_OVERRIDE override
-
 // A macro to disallow the copy constructor and operator= functions.
 // This should be used in the private: declarations for a class.
 #define MOJO_DISALLOW_COPY_AND_ASSIGN(TypeName) \