hydra-module.nix: Remove the useWAL option

This is not a Hydra option but a Nix option so it shouldn't be here.

Created by  Eelco Dolstra  on November 6, 2013
25DXUDOJ5V5UJEVJVZAXT4FV7TOKO6CQRERRA2BSSFFPAEPTRIJAC
Change contents