Move assets/icons to the main repository

Only the assets/publish folder was moved in #137219, which broke the
`make icons_geom` command for generating toolbar icons since the
submodule is now gone.

This patch moves the icons folder into a new
`release/datafiles/icons_blend` folder and updates
`blender_icons_geom_update.py` to fix the problem.

Pull Request: https://projects.blender.org/blender/blender/pulls/142639

release/datafiles/blender_icons_geom_update.py

@@ -59,7 +59,7 @@ def main() -> int:
                 blender_bin = blender_app_path
 
     icons_blend = (
-        os.path.join(ROOTDIR, "release", "datafiles", "assets", "icons", "toolbar.blend"),
+        os.path.join(ROOTDIR, "release", "datafiles", "icons_blend", "toolbar.blend"),
     )
 
     def names_and_time_from_path(path: str) -> Iterator[tuple[str, float]]: