cred: groupmember(): Remove tolerance for empty 'cr_groups'

This tolerance had been introduced in commit caa309c8811d ("nfsd: Fix
handling of credentials with cr_ngroups == 0", on 2024/10/21).

Now that NFS has been modified in the previous commit (cfbe7a62dc62,
"nfs, rpc: Ensure kernel credentials have at least one group") to rule
out credentials with empty 'cr_groups' (and thus, no 'cr_gid'), remove
it back.

This basically reverts the above-mentioned commit except for the not
directly related modifications it contains, which are still valid.

Discussed with: rmacklem (by mail)
Approved by: markj (mentor)
MFC after: 3 days