Get rid of <nixpkgs/nixos> import for sshd

[Atemu]

This has obvious issues but the reason why it's used is that we need a sensible default config to run the sshd with. We need some other way to achieve that.

Perhaps sshd has sensible defaults upstream? That'd be ideal.

With import:

$ hyperfine nix-instantiate
Benchmark 1: nix-instantiate
  Time (mean ± σ):      2.289 s ±  0.104 s    [User: 1.629 s, System: 0.352 s]
  Range (min … max):    2.094 s …  2.385 s    10 runs

Import stubbed:

$ hyperfine nix-instantiate
Benchmark 1: nix-instantiate
  Time (mean ± σ):      1.507 s ±  0.070 s    [User: 1.021 s, System: 0.244 s]
  Range (min … max):    1.420 s …  1.658 s    10 runs