hydra-module: Don't rely on su being available.

The su binary is now in a separate output of the shadow package and
isn't included in the main output path anymore.

But instead of changing the call to use pkgs.su, we're now entirely
dropping the dependency because systemd is already able to execute
processes under a specific user by itself.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Merge pull request #152 from aszlig/module-without-su

hydra-module: Don't rely on su being available in shadow.