Removed short aliases for 16-bit pixel formats

These were potentially misleading in the same way the RGB888/BGR888 aliases were

Fixes https://github.com/libsdl-org/SDL/issues/4994

test/testyuv.c

@@ -351,7 +351,7 @@ int main(int argc, char **argv)
                 yuv_format = SDL_PIXELFORMAT_NV21;
                 consumed = 1;
             } else if (SDL_strcmp(argv[i], "--rgb555") == 0) {
-                rgb_format = SDL_PIXELFORMAT_RGB555;
+                rgb_format = SDL_PIXELFORMAT_XRGB1555;
                 consumed = 1;
             } else if (SDL_strcmp(argv[i], "--rgb565") == 0) {
                 rgb_format = SDL_PIXELFORMAT_RGB565;