nixos/…/swap.nix: don't create a LUKS header for randomEncryption

Creating and then erasing the key relies on the disk erasing data
correctly, and otherwise allows attackers to simply decrypt swap just
using "secretkey". We don't actually need a LUKS header, so we can save
ourselves some pointless disk writes and identifiability.

In addition, I wouldn't have made the awful mistake of backing up my swap partition's LUKS header instead of my zpool's. May my data rest in peace.