FIX: Always preload admin plugin list for admin in sidebar (#25606)

When we show the links to installed plugins in the admin
sidebar (for plugins that have custom admin routes) we were
previously only doing this if you opened /admin, not if you
navigated there from the main forum. We should just always
preload this data if the user is admin.

This commit also changes `admin_sidebar_enabled_groups` to
not be sent to the client as part of ongoing efforts to
not check groups on the client, since not all a user's groups
may be serialized.

app/controllers/admin/admin_controller.rb

@@ -7,13 +7,4 @@ class Admin::AdminController < ApplicationController
   def index
     render body: nil
   end
-
-  private
-
-  def preload_additional_json
-    store_preloaded(
-      "enabledPluginAdminRoutes",
-      MultiJson.dump(Discourse.plugins_sorted_by_name.filter_map(&:admin_route)),
-    )
-  end
 end