smbd: remove unused current_user.{need,done}_chdir stacking

smbd: remove unused current_user.{need,done}_chdir stacking

All become_*()/unbecome_*() functions don't alter the working
directory. Only change_to_user_and_service*() does.

Pair-Programmed-With: Ralph Boehme <slow@samba.org>

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Sep 11 21:16:57 UTC 2019 on sn-devel-184