xorg-server variants: eradicate symlink to /var/tmp

- Fixes #19673; it caused problems in combination with buildEnv.
- As noted, X falls back to /tmp:
  https://github.com/NixOS/nixpkgs/issues/19673#issuecomment-258871876
- Removing the directory is still required, as X would attempt to write
  into it if allowed - and probably succeed in case the user set
  nix.readOnlyStore = false; (X runs as root).
- Archeology link: 9d156931.