Migration: Remove superfluous imports

priv/repo/migrations/20200603105113_add_unique_index_to_chat_message_references.exs

@@ -1,13 +1,6 @@
 defmodule Pleroma.Repo.Migrations.BackfillChatMessageReferences do
   use Ecto.Migration
 
-  alias Pleroma.Chat
-  alias Pleroma.ChatMessageReference
-  alias Pleroma.Object
-  alias Pleroma.Repo
-
-  import Ecto.Query
-
   def change do
     create(unique_index(:chat_message_references, [:object_id, :chat_id]))
   end