[nix-book] Remove linuxPackages_latest to fix boot I/O timeouts

After downgrade from nixos-unstable to 25.05, nix-book experienced
I/O timeouts during early boot due to incompatible kernel drivers
for Thunderbolt/VMD hardware. Using stable kernel resolves the issue.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

machines/nix-book/configuration.nix

@@ -37,9 +37,6 @@
   networking.hostName = "nix-book"; # Define your hostname.
   # networking.wireless.enable = true;  # Enables wireless support via wpa_supplicant.
 
-  boot.kernelPackages = pkgs.linuxPackages_latest;
-
-
   # Enable networking
   networking.networkmanager.enable = true;