archive/zip: remove an out of date TODO
archive/zip: remove an out of date TODO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/48857
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>
Run-TryBot: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

josharian/go,polyverse-security/go,vsekhar/elastic-go,cloudflare/go,codestation/go,Cofyc/go,vsekhar/elastic-go,golang/go,josharian/go,golang/go,Cofyc/go,golang/go,Cofyc/go,CAFxX/go,josharian/go,vsekhar/elastic-go,golang/go,nawawi/go,nawawi/go,Cofyc/go,polyverse-security/go,josharian/go,polyverse-security/go,akutz/go,cloudflare/go,Cofyc/go,vsekhar/elastic-go,codestation/go,Cofyc/go,polyverse-security/go,akutz/go,codestation/go,golang/go,CAFxX/go,codestation/go,CAFxX/go,josharian/go,christopher-henderson/Go,golang/go,Cofyc/go,codestation/go,nawawi/go,nawawi/go,akutz/go,akutz/go,CAFxX/go,vsekhar/elastic-go,akutz/go,nawawi/go,akutz/go,golang/go,akutz/go,nawawi/go,CAFxX/go,Cofyc/go,cloudflare/go,CAFxX/go,polyverse-security/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,vsekhar/elastic-go,akutz/go,nawawi/go,polyverse-security/go,josharian/go,cloudflare/go,codestation/go,vsekhar/elastic-go,polyverse-security/go,josharian/go,codestation/go,nawawi/go,CAFxX/go,josharian/go,polyverse-security/go,codestation/go,vsekhar/elastic-go