archive/zip: remove an out of date TODO

It had been implemented. Refer to https://golang.org/pkg/archive/zip/#FileHeader.Comment

Change-Id: I81572562c3111b58189baa3510b9ba688fe269c9
Reviewed-on: https://go-review.googlesource.com/48857Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/zip/writer.go

@@ -14,8 +14,6 @@ import (
 	"unicode/utf8"
 )
 
-// TODO(adg): support zip file comments
-
 // Writer implements a zip file writer.
 type Writer struct {
 	cw          *countWriter