nix-builder-vm: disable auto-optimise-store

This option is associated with corruption[1, 2] and is disabled by
default in Nix as it can cause significant slowdowns[3].

[1]: https://github.com/NixOS/nix/issues/7273
[2]: https://github.com/LnL7/nix-darwin/pull/1152
[3]: https://github.com/NixOS/nix/issues/462#issuecomment-378189808

nixos/modules/profiles/nix-builder-vm.nix

@@ -127,8 +127,6 @@ in
     system.disableInstallerTools = true;
 
     nix.settings = {
-      auto-optimise-store = true;
-
       min-free = cfg.min-free;
 
       max-free = cfg.max-free;