cogl-texture: Remove the gl_handle from CoglTextureSliceCallback

There's no longer any need to use the GL handle in the callback for
_cogl_texture_foreach_sub_texture_in_region because it can now work in
terms of primitive cogl textures so it has now been removed. This
would be helpful if we ever want to make the foreach function public
so that apps could implement their own primitives using sliced
textures.

cogl/cogl-texture-rectangle.c

@@ -107,8 +107,6 @@ _cogl_texture_rectangle_manual_repeat_cb (const float *coords,
   slice_coords[3] *= data->tex_rect->height;
 
   data->callback (COGL_TEXTURE (data->tex_rect),
-                  data->tex_rect->gl_texture,
-                  GL_TEXTURE_RECTANGLE_ARB,
                   slice_coords,
                   coords,
                   data->user_data);