Remove MSWGetFocusHwnd() helper

It doesn't seem worth having it when it's only used in a couple of
places in a single file, unlike GetHwnd() which is used in dozens of
places across entire wxMSW.

include/wx/msw/private.h

@@ -838,7 +838,6 @@ private:
 // ---------------------------------------------------------------------------
 
 #define GetHwnd()               ((HWND)GetHWND())
-#define MSWGetFocusHwnd()       ((HWND)MSWGetFocusHWND())
 #define GetHwndOf(win)          ((HWND)((win)->GetHWND()))
 // old name
 #define GetWinHwnd              GetHwndOf