hydra: remove some layout, test

Dependencies

• [2] YQWH4POV * Simplify.
• [3] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 75
  [2.1937]→[2.1937:1952](∅→∅)
  − ''; # */
  [2.1937]

  [3.1304]
  + '';