Blender Git Loki

Cleanup: asset catalog service, remove obsolete `write_to_disk` function

Remove `AssetCatalogService::write_to_disk()` function. It has been
superseded by `write_to_disk_on_blendfile_save()`; the handful of test
functions that called the old function have been adjusted to use the
new one.

No functional changes to Blender itself.