DEV: Remove enable_experimental_hashtag_autocomplete logic (#22820)

This commit removes any logic in the app and in specs around
enable_experimental_hashtag_autocomplete and deletes some
old category hashtag code that is no longer necessary.

It also adds a `slug_ref` category instance method, which
will generate a reference like `parent:child` for a category,
with an optional depth, which hashtags use. Also refactors
PostRevisor which was using CategoryHashtagDataSource directly
which is a no-no.

Deletes the old hashtag markdown rule as well.

lib/post_revisor.rb

@@ -160,18 +160,14 @@ class PostRevisor
       user,
       I18n.t(
         "topic_category_changed",
-        from: category_name_raw(old_category),
-        to: category_name_raw(new_category),
+        from: "##{old_category.slug_ref}",
+        to: "##{new_category.slug_ref}",
       ),
       post_type: Post.types[:small_action],
       action_code: "category_changed",
     )
   end
 
-  def self.category_name_raw(category)
-    "##{CategoryHashtagDataSource.category_to_hashtag_item(category).ref}"
-  end
-
   def self.create_small_action_for_tag_changes(topic:, user:, added_tags:, removed_tags:)
     return if !SiteSetting.create_post_for_category_and_tag_changes