hosts: poseidon: remove deprecated grub version

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/3a70dd92993182f8e514700ccf5b1ae9fc8a3b8d' (2023-05-31)
  → 'github:NixOS/nixpkgs/e7603eba51f2c7820c0a182c6bbb351181caa8e7' (2023-06-04)

flake.lock

@@ -128,11 +128,11 @@
     },
     "nixpkgs_2": {
       "locked": {
-        "lastModified": 1685533922,
-        "narHash": "sha256-y4FCQpYafMQ42l1V+NUrMel9RtFtZo59PzdzflKR/lo=",
+        "lastModified": 1685865905,
+        "narHash": "sha256-XJZ/o17eOd2sEsGif+/MQBnfa2DKmndWgJyc7CWajFc=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "3a70dd92993182f8e514700ccf5b1ae9fc8a3b8d",
+        "rev": "e7603eba51f2c7820c0a182c6bbb351181caa8e7",
         "type": "github"
       },
       "original": {

hosts/poseidon/default.nix

@@ -19,7 +19,6 @@ in {
 
   # Use the GRUB 2 boot loader.
   boot.loader.grub.enable = true;
-  boot.loader.grub.version = 2;
   boot.loader.grub.device = "/dev/sda"; # or "nodev" for efi only
 
   boot.supportedFilesystems = ["btrfs"];