nixos: remove the grsecurity GID

This GID was used to exempt users from Grsecurity's
`/proc` restrictions; we now prefer to rely on
`security.hideProcessInformation`, which uses the `proc` group
for this purpose.  That leaves no use for the grsecurity GID.

More generally, having only a single GID to, presumably, serve as the
default for all of grsecurity's GID based exemption/resriction schemes
would be problematic in any event, so if we decide to enable those
grsecurity features in the future, more specific GIDs should be added.