dir.c: drop unused "untracked" from treat_path_fast()

dir.c: drop unused "untracked" from treat_path_fast()

We don't use the untracked_cache_dir parameter that is passed in, but
instead look at the untracked_cache_dir inside the cached_dir struct we
are passed. It's been this way since the introduction of
treat_path_fast() in 91a2288b5f (untracked cache: record/validate dir
mtime and reuse cached output, 2015-03-08).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>