_pkg.dev: drop Checksum package

Duplicates hash.Checksum slightly and doesn't seem to fit into master
well.

_pkg.dev/wire/util/Checksum/checksum.go

@@ -1,33 +0,0 @@
-package checksum
-
-import (
-	"bytes"
-	"encoding/binary"
-
-	"github.com/CityOfZion/neo-go/pkg/crypto/hash"
-)
-
-// Compare calculates the checksum of b
-// then compares it with the `have` checksum passed as a parameter
-func Compare(have uint32, b []byte) bool {
-	want := FromBytes(b)
-	return have == want
-}
-
-// FromBuf calculates the checksum of a buffer
-func FromBuf(buf *bytes.Buffer) uint32 {
-
-	return FromBytes(buf.Bytes())
-}
-
-// FromBytes calculates the checksum of a byte slice
-func FromBytes(buf []byte) uint32 {
-	b, err := hash.DoubleSha256(buf)
-
-	if err != nil {
-		return 0
-	}
-
-	// checksum := SumSHA256(SumSHA256(buf.Bytes()))
-	return binary.LittleEndian.Uint32(b.Bytes()[:4])
-}