Remove unused SDL_TextureModulate enum

Fixes https://github.com/libsdl-org/SDL/issues/6387

include/SDL3/SDL_render.h

@@ -115,16 +115,6 @@ typedef enum
     SDL_TEXTUREACCESS_TARGET     /**< Texture can be used as a render target */
 } SDL_TextureAccess;
 
-/**
- * The texture channel modulation used in SDL_RenderTexture().
- */
-typedef enum
-{
-    SDL_TEXTUREMODULATE_NONE = 0x00000000,     /**< No modulation */
-    SDL_TEXTUREMODULATE_COLOR = 0x00000001,    /**< srcC = srcC * color */
-    SDL_TEXTUREMODULATE_ALPHA = 0x00000002     /**< srcA = srcA * alpha */
-} SDL_TextureModulate;
-
 /**
  * Flip constants for SDL_RenderTextureRotated
  */