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/testjoystick.c

@@ -186,7 +186,7 @@ loop(void *arg)
                 (event.key.keysym.sym != SDLK_AC_BACK)) {
                 break;
             }
-            SDL_FALLTHROUGH;
+            /* Fall through to signal quit */
         case SDL_FINGERDOWN:
         case SDL_MOUSEBUTTONDOWN:
         case SDL_QUIT: