nixos/impermanence: remove ssh host key persistence

The SSH host key files are already defined in the OpenSSH module, so
there is no need to persist them with impermanence.nix.

nixos/impermanence.nix

@@ -72,10 +72,6 @@
     ];
     files = [
       "/etc/machine-id"
-      "/etc/ssh/ssh_host_ed25519_key"
-      "/etc/ssh/ssh_host_ed25519_key.pub"
-      "/etc/ssh/ssh_host_rsa_key"
-      "/etc/ssh/ssh_host_rsa_key.pub"
     ];
   };