Remove mention of github-markup in Wiki clone help

We drop the mention of `gem install github-markup` for the following
reasons:

1. Most of our custom markup logic is bundled in the GitLab Banzai
rendering.
2. github-markup is no longer maintained, and we actually use our own
fork: gitlab-markup.
3. Installing gollum automatically installs Kramdown and github-markup.

app/views/projects/wikis/git_access.html.haml

@@ -18,11 +18,6 @@
   %pre.dark
     :preserve
       gem install gollum
-  %p
-    = (s_("WikiClone|It is recommended to install %{markdown} so that GFM features render locally:") % { markdown: "<code>github-markdown</code>" }).html_safe
-  %pre.dark
-    :preserve
-      gem install github-markdown
 
   %h3= s_("WikiClone|Clone your wiki")
   %pre.dark

changelogs/unreleased/sh-use-gitlab-markdown-in-wiki.yml

+---
+title: Remove mention of github-markup in Wiki clone help
+merge_request: 30962
+author:
+type: other

locale/gitlab.pot

@@ -23910,9 +23910,6 @@ msgstr ""
 msgid "WikiClone|Install Gollum"
 msgstr ""
 
-msgid "WikiClone|It is recommended to install %{markdown} so that GFM features render locally:"
-msgstr ""
-
 msgid "WikiClone|Start Gollum and edit locally"
 msgstr ""