nixos: remove redundant services.dovecot2.package option

Instead of using this option, please modify the dovecot package by means of an
override. For example:

  nixpkgs.config.packageOverrides = super: {
    dovecot = super.dovecot.override { withPgSQL = true; };
  };

Closes https://github.com/NixOS/nixpkgs/issues/14097.