transmogrifier: use User.delete() instead of handrolled user deletion code for remote users

Closes #1104

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -656,20 +656,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
       nil ->
         case User.get_cached_by_ap_id(object_id) do
           %User{ap_id: ^actor} = user ->
-            {:ok, followers} = User.get_followers(user)
-
-            Enum.each(followers, fn follower ->
-              User.unfollow(follower, user)
-            end)
-
-            {:ok, friends} = User.get_friends(user)
-
-            Enum.each(friends, fn followed ->
-              User.unfollow(user, followed)
-            end)
-
-            User.invalidate_cache(user)
-            Repo.delete(user)
+            User.delete(user)
 
           nil ->
             :error