Don't set $TZ

Now that Java is happy with our /etc/localtime, there is no reason to
set $TZ anymore.  (See 945849b86f, 279248f6c5, 1b5e860f65607b4cc7de4b6b5db95460cf144526.)

Fixes #1463.

nixos/modules/config/timezone.nix

@@ -25,7 +25,6 @@ with pkgs.lib;
   config = {
 
     environment.variables.TZDIR = "/etc/zoneinfo";
-    environment.variables.TZ = config.time.timeZone;
 
     environment.etc.localtime =
       { source = "${pkgs.tzdata}/share/zoneinfo/${config.time.timeZone}";