raft: leave TODO about leaving StateSnapshot
raft: leave TODO about leaving StateSnapshot

The condition is overly strict, which has popped up in CockroachDB
recently.

rnaveiras/etcd,etcd-io/etcd,mqliang/etcd,coreos/etcd,coreos/etcd,mqliang/etcd,mqliang/etcd,cockroachdb/etcd,li-ang/etcd,etcd-io/etcd,ajityagaty/etcd,ajityagaty/etcd,rnaveiras/etcd,rnaveiras/etcd,etcd-io/etcd,gyuho/etcd,mkumatag/etcd,gyuho/etcd,dannysauer/etcd,ajityagaty/etcd,dannysauer/etcd,mkumatag/etcd,mkumatag/etcd,li-ang/etcd,cockroachdb/etcd