Remove CheckSum from utils; replace with a TeeReader

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)

utils/checksum.go

@@ -1,24 +0,0 @@
-package utils
-
-import (
-	"encoding/hex"
-	"hash"
-	"io"
-)
-
-type CheckSum struct {
-	io.Reader
-	Hash hash.Hash
-}
-
-func (cs *CheckSum) Read(buf []byte) (int, error) {
-	n, err := cs.Reader.Read(buf)
-	if err == nil {
-		cs.Hash.Write(buf[:n])
-	}
-	return n, err
-}
-
-func (cs *CheckSum) Sum() string {
-	return hex.EncodeToString(cs.Hash.Sum(nil))
-}