This is safe since all calls to `ppNix` are made using `.text()`.

 * https://github.com/NixOS/nixos-homepage/blob/f74049df65db247ce156022155a966c272ffb96c/nixos/options.tt#L324-L328

And *anyway* the previous escapes were done only one time, making it
trivial to bypass

```
> "<<<<<".replace("<", "&gt;")
→ "&gt;<<<<"
```

This has further been manually verified (with an hand-modified options.json) to
not cause opportunities for HTML injection.

Removes useless escapes from `ppNix` in options page.