nixos: generate infodirs directly in system-path

`man 1 info` says:

   The first non-option argument, if present, is the menu entry to
   start from; it is searched for in all `dir' files along INFOPATH.
   If it is not present, info merges all `dir' files and shows the
   result. Any remaining arguments are treated as the names of menu
   items relative to the initial node visited.

Which means that this does what previous programs/info did and #8519
(on-the-fly infodir generation for Emacs) wanted to do, but for both
programs.

nixos/modules/programs/info.nix

@@ -1,38 +0,0 @@
-{config, pkgs, ...}:
-
-let
-
-  texinfo = pkgs.texinfoInteractive;
-
-  # Quick hack to make the `info' command work properly.  `info' needs
-  # a "dir" file containing all the installed Info files, which we
-  # don't have (it would be impure to have a package installation
-  # update some global "dir" file).  So this wrapper script around
-  # "info" builds a temporary "dir" file on the fly.  This is a bit
-  # slow (on a cold cache) but not unacceptably so.
-  infoWrapper = pkgs.writeScriptBin "info"
-    ''
-      #! ${pkgs.stdenv.shell}
-
-      dir=$(mktemp --tmpdir -d "info.dir.XXXXXX")
-
-      if test -z "$dir"; then exit 1; fi
-
-      trap 'rm -rf "$dir"' EXIT
-
-      shopt -s nullglob
-
-      for i in $(IFS=:; echo $INFOPATH); do
-          for j in $i/*.info; do
-              ${texinfo}/bin/install-info --quiet $j $dir/dir
-          done
-      done
-
-      INFOPATH=$dir:$INFOPATH ${texinfo}/bin/info "$@"
-    ''; # */
-
-in
-
-{
-  environment.systemPackages = [ infoWrapper texinfo ];
-}