Revert "Allow WM_SYSKEYDOWN to still be processed by MSWDefWindowProc, even if"

This reverts commit e771b7e4ac.

Don't always pass WM_SYSKEYDOWN to the system for processing as this can
be undesirable: e.g. inside a dialog, any Alt-X key presses with X not
being a mnemonic character result in a beep from IsDialogMessage().
Handle events for the system keys in the same way as for the normal
ones, i.e. let the system process them only if they're not handled by
the application.

This is incompatible with the previous wxMSW behaviour, but compatible
with the other ports and makes more sense, so it seems to be worth it,
on balance.

src/msw/window.cpp

@@ -3258,8 +3258,6 @@ wxWindowMSW::MSWHandleMessage(WXLRESULT *result,
                             processed = HandleWindowEvent(event);
                 }
             }
-            if (message == WM_SYSKEYDOWN)  // Let Windows still handle the SYSKEYs
-                processed = false;
             break;
 
         case WM_SYSKEYUP: