Turn on SFTP - scp defaults to using it now.

nixos/modules/ssh.nix

@@ -25,9 +25,6 @@
     services.openssh = {
       enable = true;
 
-      # We don't use SFTP for anything.  No reason to expose it.
-      allowSFTP = false;
-
       # We only allow key-based authentication.
       kbdInteractiveAuthentication = false;
       passwordAuthentication = false;