use another stream function in migration

priv/repo/migrations/20200825061316_move_activity_expirations_to_oban.exs

@@ -12,9 +12,8 @@ def change do
     from(e in "activity_expirations",
       select: %{id: e.id, activity_id: e.activity_id, scheduled_at: e.scheduled_at}
     )
-    |> Pleroma.RepoStreamer.chunk_stream(500)
-    |> Stream.each(fn expirations ->
-      Enum.each(expirations, fn expiration ->
+    |> Pleroma.Repo.stream()
+    |> Enum.each(fn expiration ->
       with {:ok, expires_at} <- DateTime.from_naive(expiration.scheduled_at, "Etc/UTC") do
         Pleroma.Workers.PurgeExpiredActivity.enqueue(%{
           activity_id: FlakeId.to_string(expiration.activity_id),
@@ -23,7 +22,6 @@ def change do
         })
       end
     end)
-    end)
     |> Stream.run()
   end
 end