Remove unnecessary CommandStateChangeConstants enum redeclaration.

This declaration conflicted with the one in the latest MinGW-W64 headers and
wasn't needed anyhow as we don't use neither the enum itself nor its members
in our code, so just remove it to fix the build in 64 bits with MinGW-W64 4.8.

This is the backport of r75669 from trunk.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75775 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/missing.h

@@ -655,21 +655,6 @@ typedef struct
 
 #ifndef MUI_LANGUAGE_NAME
 #define MUI_LANGUAGE_NAME 0x8
-#endif
-
-//We need to check if we are using MinGW or mingw-w64 as their
-//definitions are different
-
-#ifdef __MINGW32__
-#include <_mingw.h>
-#endif
-
-#ifdef __MINGW32_TOOLCHAIN__
-typedef enum CommandStateChangeConstants {
-    CSC_UPDATECOMMANDS = (int) 0xFFFFFFFF,
-    CSC_NAVIGATEFORWARD = 0x1,
-    CSC_NAVIGATEBACK = 0x2
-} CommandStateChangeConstants;
 #endif
 
  /*