Integrate formatter into flake.nix

[georgeee]

1. Enable nix fmt by integrating nixfmt into flake
2. Remove two nix files that are no longer used
3. Run formatter against repository

Choice of nix formatter out of three candidates (alejandra, nixpkgs-fmt, nixfmt) was purely technical, minimizing the number of lines in the diff produced by the formatter.

Explain how you tested your changes:

• Executed the formatter
• Built nix

Checklist:

• [x] Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• [x] Modified the current draft of release notes with details on what is completed or incomplete within this project
• [x] Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• [x] Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• [x] All tests pass (CI will check this if you didn't)
• [x] Serialized types are in stable-versioned modules
• [x] Does this close issues? None