Remove the parts about the effective GID being stored as the first
element of the groups array, as this is generally not the case now after
commit be1f7435ef21 ("kern: start tracking cr_gid outside of
cr_groups[]") (some credentials export structures still have the old
layout, and need to do the 'ngroups_max + 1' computation, but given the
computation needed to obtain a size in bytes, this detail is not really
important here).

Remove the part about 32-bit machines, avoiding another modification
when support for them is removed.

No functional change.

Sponsored by: The FreeBSD Foundation