nsswitch: Remove IRIX support

According to wikipedia, IRIX has seen the last patch update in August 2006. As
of now, www.sgi.com is unreachable. Probably this code has not been built in
years. If someone wants to revive it, it can be found in the git history.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
