posts: fix expunging posts not removing image from iqdb.

`File.exists?(preview_file_path)` is always false because the file has
already been deleted by this point. Remove the check entirely.

app/models/post.rb

@@ -1713,9 +1713,7 @@ class Post < ActiveRecord::Base
     end
 
     def remove_iqdb_async
-      if File.exists?(preview_file_path) && Post.iqdb_enabled?
-        Post.iqdb_sqs_service.send_message("remove\n#{id}")
-      end
+      Post.remove_iqdb(id)
     end
 
     def update_iqdb