remove reference to non-existing nix-common.css

Dependencies

• [2] FKK2ZSTL * Split the CSS.
• [3] BA46C5LN * Pretty-print the logs.
• [4] QL55ECJ6 - adapted ui for hydra, more in line with nixos.org website
• [*] J5UVLXOK * Start of a basic Catalyst web interface.

Change contents

• edit in src/root/layout.tt at line 15
  [2.76]→[2.76:156](∅→∅)
  − <link rel="stylesheet" href="/static/css/nix-common.css" type="text/css" />