fix: saving ckpt error when ckpt folder has some folder inside

[Crescent-Saturn]

When save_only_latest_checkpoint is True, it tried to delete everything in the chekpoint folder before saving the latest checkpoint.

Since it is supposed that the nerfstudio_models only has checkpoint files inside, when there are some other customized saved folders inside, there will be an error.

This change will list *.ckpt files only instead of everything (include folder) to be compared with latest checkpoint file before deleting.

Thanks for your consideration