Switch to User.delete for rm_user task

lib/mix/tasks/rm_user.ex

 defmodule Mix.Tasks.RmUser do
   use Mix.Task
   import Mix.Ecto
-  alias Pleroma.{Repo, User}
+  alias Pleroma.User
 
   @shortdoc "Permanently delete a user"
   def run([nickname]) do
     ensure_started(Repo, [])
 
     with %User{local: true} = user <- User.get_by_nickname(nickname) do
-      Repo.delete!(user)
+      user.delete()
     end
   end
 end