Remove compile-time checks for always defined WM_XXX in wxMSW

The symbols in question are present at least since XP, and most of them since
Windows 2000, so remove preprocessor checks for them, they shouldn't be needed
when any remotely current compiler/SDK.

src/msw/anybutton.cpp

@@ -90,10 +90,6 @@ using namespace wxMSWImpl;
     #endif
 #endif // wxUSE_UXTHEME
 
-#ifndef WM_THEMECHANGED
-    #define WM_THEMECHANGED     0x031A
-#endif
-
 #ifndef ODS_NOACCEL
     #define ODS_NOACCEL         0x0100
 #endif