s4-rpc_server/drsuapi: Remove rudundant check for valid and non-NULL ncRoot_dn

s4-rpc_server/drsuapi: Remove rudundant check for valid and non-NULL ncRoot_dn

This check was valuable before aee2039e63ceeb5e69a0461fb77e0f18278e4dc4
but now only checks things we know to be true, as the value has come
from Samba via drs_ObjectIdentifier_to_dn_and_nc_root() either on this
or a previous call.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15401

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit 0550e469eda4022659718ae9a56f5deaa9f9a307)