Revert "Add and use SDL_FALLTHROUGH for fallthroughs"

This reverts commit 66a08aa391.

This causes problems with older compilers:
https://github.com/libsdl-org/SDL/pull/4791#issuecomment-966630997

src/video/windows/SDL_windowsevents.c

@@ -729,7 +729,6 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
             }
         }
         /* don't break here, fall through to check the wParam like the button presses */
-        SDL_FALLTHROUGH;
     case WM_LBUTTONUP:
     case WM_RBUTTONUP:
     case WM_MBUTTONUP: