Revert "add nixpkgs version option"

This reverts commit b777f9e6df.

nixos-generate

@@ -15,7 +15,6 @@ run=
 nix_args=(
   "$libexec_dir/nixos-generate.nix"
 )
-nixpkgs_ver=
 
 ## Functions
 
@@ -36,7 +35,6 @@ Options:
 * --system: specify the target system (eg: x86_64-linux)
 * --cores : to control the maximum amount of parallelism. (see nix-build documentation)
 * -I KEY=VALUE: Add a key to the Nix expression search path.
-* --nixpkgs_ver: set the release version of nixpkgs
 USAGE
 }
 
@@ -93,10 +91,6 @@ while [[ $# -gt 0 ]]; do
     -I)
       nix_args+=(-I "$2")
       shift
-      ;;
-	--nixpkgs_ver)
-      nixpkgs_ver=$2
-      shift
       ;;
     *)
       abort "unknown option $1"
@@ -109,14 +103,9 @@ if [[ -z $format_path ]]; then
   abort "missing format. use --help for more details"
 fi
 
-if [[ -n $nixpkgs_ver ]]; then
-  args+=(--nixpkgs_ver "$nixpkgs_ver")
-fi
-
 nix_args+=(
   -I "nixos-config=$configuration"
   -I "format-config=$format_path"
-  -I "nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-$nixpkgs_ver.tar.gz"
 )
 
 if [[ -n $target_system ]]; then
@@ -129,7 +118,7 @@ fi
 
 formatAttr=$(nix-instantiate "${nix_args[@]}" --eval --json -A config.formatAttr | jq -r .)
 
-out=$(nix-build "${nix_args[@]}" -A "config.system.build.$formatAttr")
+out=$(nix-build "${nix_args[@]}" --no-out-link -A "config.system.build.$formatAttr")
 
 if [[ -z $run ]]; then
   # show the first file, ignoring nix-support