nfsd: nfsd_excred(): Remove an obsolete comment

This comment has been obsoleted by commit be1f7435ef21 ("kern: start
tracking cr_gid outside of cr_groups[].").

No functional change.

MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52264

(cherry picked from commit 5fa6ca8b8affc8d948687c4ede7aeed297ae2dc0)