rafthttp: remove the unncessary TODO

The issue is not caused by this code, but by reading snapshot
from disk. etcd assumes the snapshot of v3 kv should live in
memory. If not, etcd does not work well anyway.

rafthttp/snapshot_sender.go

@@ -114,9 +114,6 @@ func (s *snapshotSender) post(req *http.Request) (err error) {
 	result := make(chan responseAndError, 1)
 
 	go func() {
-		// TODO: the snapshot could be large and eat up all resources when writing
-		// it out. Send it block by block and rest some time between to give the
-		// time for main loop to run.
 		resp, err := s.tr.RoundTrip(req)
 		if err != nil {
 			result <- responseAndError{resp, nil, err}