subsys/fs: remove ambiguity in readdir results

Existing file system implementations do not provide the special "."
(current) and ".." (parent) directory entries in the readdir results.
littlefs does.

Remove these entries in the abstraction layer.  This simplifies code in
higher level consumers that aren't prepared to see them.  Consumers like
FUSE that need them can put them back without having to worry about
conflicts.

Closes issue #17951

Signed-off-by: Peter A. Bigot <pab@pabigot.com>