DEV: Move a plugin related system spec to footnote plugin (#31230)

In a previous PR, I introduced this system spec that checks that a sidebar link is auto-generated for certain plugins.

This causes problems, because the core test suite can be run with plugins either enabled or disabled, causing flaky tests.

spec/system/admin_sidebar_navigation_spec.rb

@@ -337,19 +337,4 @@ describe "Admin | Sidebar Navigation", type: :system do
       ],
     )
   end
-
-  it "adds auto-generated plugin links for plugins with settings" do
-    skip if Discourse.plugins.empty?
-
-    SiteSetting.enable_markdown_footnotes = true
-
-    visit("/admin")
-
-    sidebar.toggle_section(:plugins)
-
-    expect(page).to have_css(
-      ".sidebar-section-link-content-text",
-      text: I18n.t("js.footnote.title"),
-    )
-  end
 end