fix: Remove incorrect ./parts/fleet-data.nix import from flake.nix and use correct flake-parts structure

lib/mkFleet.nix

@@ -2,10 +2,9 @@
 # Usage: nixosConfigurations = athenix.lib.mkFleet { fleet = { ... }; hwTypes = { ... }; }
 {
   inputs,
-  fleet ? null,
-  hwTypes ? null,
+  lib,
+  config
 }:
 import ../fleet/default.nix {
-  inherit inputs;
-  inherit fleet hwTypes;
+  inherit inputs lib config;
 }