Don't put ssmtp in hydra's paths

Dependencies

• [2] ZYEFL36Q Ported/merged properties from the TUD configuration's hydra-module
• [3] KMPLMG7T hydra-module.nix: Cleanup
• [4] D3MDJONY

Change contents

• replacement in hydra-module.nix at line 202
  [2.1588]→[2.1588:1633](∅→∅)
  − path = [ pkgs.nettools pkgs.ssmtp ];
  [2.1588]

  [2.1633]
  + path = [ pkgs.nettools ];
• replacement in hydra-module.nix at line 216
  [2.2103]→[2.2103:2148](∅→∅)
  − path = [ pkgs.nettools pkgs.ssmtp ];
  [2.2103]

  [2.2148]
  + path = [ pkgs.nettools ];