Remove extra SSH jump configuration

We now have direct visibility among nodes so we don't need any extra
SSH configuration to reach them.

Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/module/ssh-hut-extern.nix

@@ -1,8 +0,0 @@
-{
-  programs.ssh.extraConfig = ''
-    Host apex ssfhead
-      HostName ssflogin.bsc.es
-    Host hut
-      ProxyJump apex
-  '';
-}