tour: remove link to korean translation

The korean translation of the tour at go-tour-ko.appspot.com has
been offline for several months, and its maintainer confirmed at
golang/go#73120 that he's no longer running it. Remove it from
the translations list.

Updates golang/tour#1039

Change-Id: I481206d4e6412ba0d2e4b6789b8998b72fa0588d
Reviewed-on: https://go-review.googlesource.com/c/website/+/694735
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: ALTree

_content/tour/welcome.article

@@ -53,7 +53,6 @@ The tour is available in other languages:
 - [[https://go-tour-cz.appspot.com/][Czech — Česky]]
 - [[https://go-tour-id2.appspot.com/][Indonesian — Bahasa Indonesia]]
 - [[https://go-tour-jp.appspot.com/][Japanese — 日本語]]
-- [[https://go-tour-ko.appspot.com/][Korean — 한국어]]
 - [[https://go-tour-pl1.appspot.com/][Polish — Polski]]
 - [[https://go-tour-lat.appspot.com/][Spanish — Español]]
 - [[https://go-tour-th.appspot.com/][Thai — ภาษาไทย]]