[x/tour] content: remove links to translations that are not online

After appspot deprecated Go 1.9, a few of the tour translations we
link to went offline. I've reported the issue on their issue trackers
6 month ago (Issue golang/tour#1039 tracks this). A few of them were
redeployed and went back online, but others are still offline (and now
appspot returns a 500 Internal Server Error status for those).

After a 6 months grace period, this change removes the links to the
translations that are still offline from the welcome page.

Updates golang/tour#1039
Updates golang/go#41219
Fixes golang/go#44382

Change-Id: I82bab0b78273b5b26dab0714ed6e6e6feaea1725
Reviewed-on: https://go-review.googlesource.com/c/tour/+/298209
Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
X-Tour-Commit: 0a3a62c5e5c0d05b1347b4156a5caa5b4e5758e3

tour/content/welcome.article

@@ -47,25 +47,15 @@ When you're ready to move on, click the [[javascript:highlightAndClick(".next-pa
 
 The tour is available in other languages:
 
-- [[https://go-tour-ar.appspot.com/][Arabic — العربية]]
 - [[https://go-tour-br.appspot.com/][Brazilian Portuguese — Português do Brasil]]
 - [[https://go-tour-ca.appspot.com/][Catalan — Català]]
 - [[https://tour.go-zh.org/][Simplified Chinese — 中文（简体）]]
-- [[https://go-tour-zh-tw.appspot.com/][Traditional Chinese — 中文（繁體）]]
 - [[https://go-tour-cz.appspot.com/][Czech — Česky]]
 - [[https://go-tour-fr.appspot.com/][French — Français]]
-- [[https://go-tour-de.appspot.com/][German — Deutsch]]
-- [[https://go-tour-he.appspot.com/][Hebrew — עִבְרִית]]
 - [[https://go-tour-id2.appspot.com/][Indonesian — Bahasa Indonesia]]
-- [[https://go-tour-ita.appspot.com/][Italian — Italiano]]
 - [[https://go-tour-jp.appspot.com/][Japanese — 日本語]]
 - [[https://go-tour-ko.appspot.com/][Korean — 한국어]]
-- [[https://go-tour-ro.appspot.com/][Romanian — Română]]
-- [[https://go-tour-ru-ru.appspot.com/][Russian - Русский]]
-- [[https://gotour-es.appspot.com/][Spanish — Español]]
 - [[https://go-tour-th.appspot.com/][Thai - ภาษาไทย]]
-- [[https://go-tour-turkish.appspot.com/][Turkish - Türkçe]]
-- [[https://go-tour-ua.appspot.com/][Ukrainian — Українська]]
 - [[https://go-tour-uz.appspot.com/][Uzbek — Ўзбекча]]
 
 Click the [[javascript:highlightAndClick(".next-page")]["next"]] button or type `PageDown` to continue.