[TechDebt:nix] Remove onnxruntime overlay from flake.nix when nixpkgs ships >= 1.24

[xilec]

Context

PR #1041 migrated to ort-sys 2.0.0-rc.12, which requires ONNX Runtime 1.24+ (API v24).
Since nixpkgs currently ships onnxruntime 1.23.2, a temporary overlay was added in flake.nix
to use Microsoft's prebuilt binaries (1.24.2).

Tracking

nixpkgs PR updating onnxruntime: NixOS/nixpkgs#499389

Action needed

Once nixpkgs (unstable) ships onnxruntime >= 1.24:

1. Remove the onnxruntime overlay from flake.nix
2. Verify the Nix build still works

[xilec]

Resolved in #1149