crypto/md5: unnecessary conversion

Fixes go lint warning.

Change-Id: I5a7485a4c8316b81e6aa50b95fe75e424f2fcedc
Reviewed-on: https://go-review.googlesource.com/109055Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/md5/md5.go

@@ -64,7 +64,7 @@ func (d *digest) MarshalBinary() ([]byte, error) {
 	b = appendUint32(b, d.s[2])
 	b = appendUint32(b, d.s[3])
 	b = append(b, d.x[:d.nx]...)
-	b = b[:len(b)+len(d.x)-int(d.nx)] // already zero
+	b = b[:len(b)+len(d.x)-d.nx] // already zero
 	b = appendUint64(b, d.len)
 	return b, nil
 }