hydra-module.nix: Remove the useWAL option

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

hydra-module.nix

@@ -108,13 +108,6 @@ in
         '';
       };
 
-      useWAL = mkOption {
-        default = true;
-        description = ''
-          Whether to use SQLite's Write-Ahead Logging, which may improve performance.
-        '';
-      };
-
       debugServer = mkOption {
         default = false;
         type = types.bool;
@@ -167,8 +160,6 @@ in
       # Online log compression makes it impossible to get the tail of
       # builds that are in progress.
       build-compress-log = false
-
-      use-sqlite-wal = ${if cfg.useWAL then "true" else "false"}
     '';
 
     systemd.services."hydra-init" =