chore: remove obsolete rustup overlay

The upstream fix (rust-lang/rustup#4372) has been merged and is
included in rustup 1.29.0, which is already available in the
locked nixpkgs. The skip-test workaround for
NixOS/nixpkgs#470496 is no longer needed; the package builds
cleanly and is fetched from cache.nixos.org.

Author: cons-tan-tan

nix/overlays/default.nix

@@ -4,8 +4,6 @@ let
   overlayFiles = [
     # LLM agents
     ./llm-agents.nix
-    # Rust tools
-    ./rustup.nix
     # Git worktree manager
     ./git-wt.nix
     # Slack automation CLI for AI agents