gloox: remove useless nulls

pkgs/development/libraries/gloox/default.nix

···
 { lib, stdenv, fetchurl
-, zlibSupport ? true, zlib ? null
-, sslSupport ? true, openssl ? null
-, idnSupport ? true, libidn ? null
+, zlibSupport ? true, zlib
+, sslSupport ? true, openssl
+, idnSupport ? true, libidn
 }:
-
-assert zlibSupport -> zlib != null;
-assert sslSupport -> openssl != null;
-assert idnSupport -> libidn != null;
 
 with lib;