fix: remove rust-toolchain.toml to avoid forcing toolchain on all developers

PR #814 added rust-toolchain.toml for Nix flake support, but this file
affects all developers using rustup, not just Nix users. The pinned
version (1.91) is outdated and conflicts with CI which tests against
stable/beta/nightly.

This change:
- Removes rust-toolchain.toml
- Updates flake.nix to specify stable Rust directly using rust-overlay
- Keeps Nix-specific config in Nix files without affecting other devs

Author: roderickvd

flake.nix

@@ -8,7 +8,9 @@
     inputs: inputs.utils.lib.eachDefaultSystem (
       system: let
         pkgs = inputs.nixpkgs.legacyPackages.${system}.extend inputs.rust-overlay.overlays.default;
-        rust = pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
+        rust = pkgs.rust-bin.stable.latest.default.override {
+          extensions = [ "rust-src" "rust-analyzer" ];
+        };
       in {
         devShell = pkgs.mkShell {
           nativeBuildInputs = with pkgs; [