Move object internal fields to a constant

lib/pleroma/constants.ex

@@ -6,4 +6,16 @@ defmodule Pleroma.Constants do
   use Const
 
   const(as_public, do: "https://www.w3.org/ns/activitystreams#Public")
+
+  const(object_internal_fields,
+    do: [
+      "likes",
+      "like_count",
+      "announcements",
+      "announcement_count",
+      "emoji",
+      "context_id",
+      "deleted_activity_id"
+    ]
+  )
 end

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -979,15 +979,7 @@ def prepare_attachments(object) do
 
   defp strip_internal_fields(object) do
     object
-    |> Map.drop([
-      "likes",
-      "like_count",
-      "announcements",
-      "announcement_count",
-      "emoji",
-      "context_id",
-      "deleted_activity_id"
-    ])
+    |> Map.drop(Pleroma.Constants.object_internal_fields())
   end
 
   defp strip_internal_tags(%{"tag" => tags} = object) do