Motivation for this change

Previously fish would generate completions from man pages if the directory they are usually stored in does not exist (given that Python exists on the $PATH, see #86195 and fish-shell/fish-shell#6956). On NixOS, the system is responsible for creating those completions. If fish creates completions itself, completions for commands that don't exist would keep being considered by fish (since they aren't automatically updated like the ones generated by NixOS).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.