nix_flake_fmt: drop legacy codepath, fix timeouts

[jfly]

What does this PR do?

Two things here, I suggest reading the commits independently:

1. Drop the legacy nix codepath
2. Fix a timeout that happens when the flake doesn't define a formatter. Reported in Smarter nix_flake_fmt condition? #349

Checklist

• If I'm adding a new builtin (linter, formatter, code action, etc.), I
  understand it should be contributed to
  nvimtools/none-ls-extras.nvim
  instead
• I've written tests for these changes

[mightyiam]

Okay, I'm not familiar with the API. Assuming you're right about _done(nil) this looks good to me and am looking forward to using it.

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[mochaaP]