nixos-generate-config: don't generate swapDevices for *files*

Up until now, the output has been the same for swap devices and swap
files:

  { device = "/var/swapfile"; }

Whereas for swap *files* it's easier to manage them declaratively in
configuration.nix:

  { device = "/var/swapfile"; size = 8192; }

(NixOS will create the swapfile, and later resize it, if the size
attribute is changed.)

With the assumption that swap files are specified in configuration.nix,
it's silly to output them to hardware-configuration.nix.