Revert "Added SDL_FLIP_DIAGONAL"

This reverts commit b9ab326982.

@rainerdeyke pointed out:
"This commit is incorrect. Flipping both horizontally and vertically is not equivalent to flipping diagonally."

include/SDL3/SDL_surface.h

@@ -80,10 +80,9 @@ typedef enum
  */
 typedef enum
 {
-    SDL_FLIP_NONE       = 0x00,     /**< Do not flip */
+    SDL_FLIP_NONE,          /**< Do not flip */
-    SDL_FLIP_HORIZONTAL = 0x01,     /**< flip horizontally */
+    SDL_FLIP_HORIZONTAL,    /**< flip horizontally */
-    SDL_FLIP_VERTICAL   = 0x02,     /**< flip vertically */
+    SDL_FLIP_VERTICAL       /**< flip vertically */
-    SDL_FLIP_DIAGONAL   = 0x03,     /**< flip diagonally (both horizontally and vertically) */
 } SDL_FlipMode;
 
 /**