hydra: remove the build-use-substitutes option

The following commit from 2016 in hydra removed the `--option
build-use-substitutes` from the hydra-queue-runner service:

```
commit ee2e9f5335c8c0288c102975b506f6b275793cfe
Author: Eelco Dolstra <edolstra@gmail.com>
Date:	Fri Oct 7 20:23:05 2016 +0200

    Update to reflect BinaryCacheStore changes

    BinaryCacheStore no longer implements buildPaths() and ensurePath(),
    so we need to use copyPath() / copyClosure().
```

It would be better if the hydra module in NixOS matches the upstream
module.

nixos/modules/services/continuous-integration/hydra/default.nix

@@ -333,7 +333,7 @@ in
           IN_SYSTEMD = "1"; # to get log severity levels
         };
         serviceConfig =
-          { ExecStart = "@${cfg.package}/bin/hydra-queue-runner hydra-queue-runner -v --option build-use-substitutes ${boolToString cfg.useSubstitutes}";
+          { ExecStart = "@${cfg.package}/bin/hydra-queue-runner hydra-queue-runner -v";
             ExecStopPost = "${cfg.package}/bin/hydra-queue-runner --unlock";
             User = "hydra-queue-runner";
             Restart = "always";