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

test/testgamecontroller.c

@@ -394,7 +394,7 @@ loop(void *arg)
             if (event.key.keysym.sym != SDLK_ESCAPE) {
                 break;
             }
-            SDL_FALLTHROUGH;
+            /* Fall through to signal quit */
         case SDL_QUIT:
             done = SDL_TRUE;
             break;