Revert to previous tag_validator behavior

This paritally reverts commit 9710063fdc
and reverts commit 7ddae61414

See thread: https://git.pleroma.social/pleroma/pleroma/-/merge_requests/4358#note_112761

lib/pleroma/web/activity_pub/object_validators/tag_validator.ex

@@ -50,13 +50,7 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.TagValidator do
   end
 
   def changeset(struct, %{"type" => "Hashtag", "name" => name} = data) do
-    name =
-      case name do
-        "#" <> name -> name
-        name -> name
-      end
-      |> String.downcase()
-
+    name = String.downcase(name)
     data = Map.put(data, "name", name)
 
     struct

test/pleroma/web/activity_pub/transmogrifier/note_handling_test.exs

@@ -200,7 +200,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier.NoteHandlingTest do
       assert match?(
                %{
                  "href" => "http://mastodon.example.org/tags/moo",
-                 "name" => "moo",
+                 "name" => "#moo",
                  "type" => "Hashtag"
                },
                Enum.at(object.data["tag"], 1)