taskserver: Remove taskserver from systemPackages

This is deliberate because using the taskd binary to configure
Taskserver has a good chance of messing up permissions.

The nixos-taskserver tool now can manage even manual configurations, so
there really is no need anymore to expose the taskd binary.

If people still want to use the taskd binary at their own risk they can
still add taskserver to systemPackages themselves.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>