feat: make ls only work for directories

Long ago, `ls` was the _object_ (merkledag) level listing tool. That's no longer
the case.

This is a [breaking change] but I don't think anyone will object.

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

core/coreapi/unixfs.go

@@ -160,9 +160,6 @@ func (api *UnixfsAPI) Ls(ctx context.Context, p coreiface.Path, opts ...options.
 	}
 
 	dir, err := uio.NewDirectoryFromNode(ses.dag, dagnode)
-	if err == uio.ErrNotADir {
-		return uses.lsFromLinks(ctx, dagnode.Links(), settings)
-	}
 	if err != nil {
 		return nil, err
 	}