spec: implement rm_file() for single file deletion

[isidentical]

No description provided.

[martindurant]

This is going in, thanks!

I wonder, when do you find it necessary to call rm_file rather than rm (which would call _bulk delete with one path) ?

[isidentical]

I wonder, when do you find it necessary to call rm_file rather than rm (which would call _bulk delete with one path) ?

DVC always uses rm_file whether deleting a single file or multiple files, and for some remotes it is more optimized than rm() since it is doing a single job and not trying to expand path / check whether it is a directory etc. And s3fs was the only one which didn't support rm_file among the stuff we migrated so far so needed that. But yes, bulk deleting is a potential optimization target for me to use when we do dvc gc, but later ofc