Fix harmless warnings about unused global variables.

Don't define variables that we never use to fix clang warnings about them.

See #15915.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75986 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/stopwatch.cpp

@@ -72,7 +72,6 @@ struct PerfCounter
 #endif // __WINDOWS__
 
 const int MILLISECONDS_PER_SECOND = 1000;
-const int MICROSECONDS_PER_MILLISECOND = 1000;
 const int MICROSECONDS_PER_SECOND = 1000*1000;
 
 } // anonymous namespace