Remove /etc/nixos/nixpkgs from $NIX_PATH

NixOS hasn't used /etc/nixos/nixpkgs for a long time, so it's time to
get rid of it.

nixos/doc/manual/release-notes/rl-unstable.xml

@@ -188,6 +188,15 @@ nix-env -f "<nixpkgs>" -iA haskellPackages.cabal-install
   </para>
 </listitem>
 
+<listitem>
+  <para>
+    The Nix expression search path (<envar>NIX_PATH</envar>) no longer
+    contains <filename>/etc/nixos/nixpkgs</filename> by default. You
+    can override <envar>NIX_PATH</envar> by setting
+    <option>nix.nixPath</option>.
+  </para>
+</listitem>
+
 </itemizedlist>
 </para>
 

nixos/modules/services/misc/nix-daemon.nix

@@ -313,7 +313,6 @@ in
         type = types.listOf types.str;
         default =
           [ "/nix/var/nix/profiles/per-user/root/channels/nixos"
-            "nixpkgs=/etc/nixos/nixpkgs"
             "nixos-config=/etc/nixos/configuration.nix"
             "/nix/var/nix/profiles/per-user/root/channels"
           ];

pkgs/tools/package-management/nixui/default.nix

@@ -2,7 +2,7 @@
 , profilePaths ? (config.nixui.profilePaths or ["/nix/var/nix/profiles"])
 , dataDir ? (config.nixui.dataDir or "/tmp")
 , configurations ? (config.nixui.configurations or ["/etc/nixos/configuration.nix"])
-, NIX_PATH ? (config.nixui.NIX_PATH or "/nix/var/nix/profiles/per-user/root/channels/nixos:nixpkgs=/etc/nixos/nixpkgs:nixos-config=/etc/nixos/configuration.nix") }:
+, NIX_PATH ? (config.nixui.NIX_PATH or "/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix") }:
 let
   version = "0.1.2";
   src = fetchgit {