remove useless check from impersonation filter
remove useless check from impersonation filter

When groupsSpecified is false, that means no other groups are added
rather than the service account groups. So this check doesn't make
any sense.

Kubernetes-commit: 0a1e24f31e5dc1a4f193a6d564ed06e2535b2830

caesarxuchao/apiserver,caesarxuchao/apiserver,kubernetes/apiserver,caesarxuchao/apiserver,kubernetes/apiserver