build: Remove traces of 'NIX_SETUID_HELPER'.

This is a followup to 0919f5f4.

* test-env.in: Remove occurrences of 'NIX_SETUID_HELPER'.

test-env.in

 #!/bin/sh
 
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2012, 2013, 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
 #
 # This file is part of GNU Guix.
 #
@@ -31,7 +31,6 @@ unset CDPATH
 
 if [ -x "@abs_top_builddir@/guix-daemon" ]
 then
-    NIX_SETUID_HELPER="@abs_top_builddir@/nix-setuid-helper" # normally unused
     NIX_STORE_DIR="@GUIX_TEST_ROOT@/store"
 
     # Do that because store.scm calls `canonicalize-path' on it.
@@ -87,10 +86,10 @@ then
     # For the (guix import snix) tests.
     NIXPKGS="@NIXPKGS@"
 
-    export NIX_IGNORE_SYMLINK_STORE NIX_STORE_DIR			\
-	NIX_LOCALSTATE_DIR NIX_LOG_DIR NIX_STATE_DIR NIX_DB_DIR		\
-	NIX_ROOT_FINDER NIX_SETUID_HELPER GUIX_BINARY_SUBSTITUTE_URL	\
-        GUIX_ALLOW_UNAUTHENTICATED_SUBSTITUTES				\
+    export NIX_IGNORE_SYMLINK_STORE NIX_STORE_DIR		\
+	NIX_LOCALSTATE_DIR NIX_LOG_DIR NIX_STATE_DIR NIX_DB_DIR	\
+	NIX_ROOT_FINDER GUIX_BINARY_SUBSTITUTE_URL		\
+        GUIX_ALLOW_UNAUTHENTICATED_SUBSTITUTES			\
         NIX_CONF_DIR XDG_CACHE_HOME NIXPKGS
 
     # Launch the daemon without chroot support because is may be