docs(snix/docs/TODO): drop rnix non-UTF8 todo

Tracked in https://git.snix.dev/snix/snix/issues/124 and
https://github.com/nix-community/rnix-parser/issues/173.

Change-Id: I5431fda1eae574d45bf6bda5d94269ba6e7fb6ba
Reviewed-on: https://cl.snix.dev/c/snix/+/30468
Autosubmit: Florian Klink <flokli@flokli.de>
Tested-by: besadii
Reviewed-by: Jonas Chevalier <zimbatm@zimbatm.com>

snix/docs/src/TODO.md

@@ -50,10 +50,6 @@ in resulting store paths.
    `anyhow`'s `.context()` gives us most of this out of the box. Maybe we can
    use that, using enums rather than `&'static str` as context in some cases?
 
-## Fixes towards correctness
- - `rnix` only supports string source files, but `NixString` uses bytes (and Nix
-   source code might be no valid UTF-8).
-
 ## Documentation
 Extend the other pages in here. Some ideas on what should be tackled:
  - Document what Tvix is, and what it is not yet. What it is now, what it is not