Remove $NIXOS_LABEL and $NIXOS_VERSION

Relying on environment variables to override configuration options is
ugly, and there is no reason for them.

nixos/modules/misc/version.nix

···
     nixosLabel = mkOption {
       type = types.str;
       description = ''
-        NixOS version name to be used in the names of generated
-        outputs and boot labels.
-
-        If you ever wanted to influence the labels in your GRUB menu,
-        this is option is for you.
-
-        Can be set directly or with <envar>NIXOS_LABEL</envar>
-        environment variable for <command>nixos-rebuild</command>,
-        e.g.:
-
-        <screen>
-        #!/bin/sh
-        today=`date +%Y%m%d`
-        branch=`(cd nixpkgs ; git branch 2>/dev/null | sed -n '/^\* / { s|^\* ||; p; }')`
-        revision=`(cd nixpkgs ; git rev-parse HEAD)`
-        export NIXOS_LABEL="$today.$branch-''${revision:0:7}"
-        nixos-rebuild switch</screen>
+        Label to be used in the names of generated outputs and boot
+        labels.
       '';
     };
 
···
     system = {
       # These defaults are set here rather than up there so that
       # changing them would not rebuild the manual
-      nixosLabel   = mkDefault (maybeEnv "NIXOS_LABEL" cfg.nixosVersion);
-      nixosVersion = mkDefault (maybeEnv "NIXOS_VERSION" (cfg.nixosRelease + cfg.nixosVersionSuffix));
+      nixosLabel   = mkDefault cfg.nixosVersion;
+      nixosVersion = mkDefault (cfg.nixosRelease + cfg.nixosVersionSuffix);
       nixosRevision      = mkIf (pathIsDirectory gitRepo) (mkDefault            gitCommitId);
       nixosVersionSuffix = mkIf (pathIsDirectory gitRepo) (mkDefault (".git." + gitCommitId));