hastd: Remove some now useless assertion

It is in fact redundant with the previous one, as the set of
supplementary groups is expected to be empty.

MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52281

(cherry picked from commit 59049f4b47ea8924ffae7c3c7b9e8c00a79285e0)