Revert "Fix relative mouse input for Unvanquished (unvanquished.net)"

This reverts commit 331859079674465a39b24f32a6a113959601dca3.

Fixes https://github.com/libsdl-org/SDL/issues/5569

src/events/SDL_mouse.c

@@ -410,7 +410,6 @@ SDL_PrivateSendMouseMotion(SDL_Window * window, SDL_MouseID mouseID, int relativ
         mouse->x = x;
         mouse->y = y;
         mouse->has_position = SDL_TRUE;
-        return 0;
     } else if (!xrel && !yrel) {  /* Drop events that don't change state */
 #ifdef DEBUG_MOUSE
         SDL_Log("Mouse event didn't change state - dropped!\n");