Fix attachment not being re-downloaded even if file is not stored (#12125)

Change the behaviour of remotable concern. Previously, it would skip
downloading an attachment if the stored remote URL is identical to
the new one. Now it would not be skipped if the attachment is not
actually currently stored by Paperclip.

app/models/account.rb

@@ -310,10 +310,9 @@ class Account < ApplicationRecord
   def save_with_optional_media!
     save!
   rescue ActiveRecord::RecordInvalid
-    self.avatar              = nil
-    self.header              = nil
-    self[:avatar_remote_url] = ''
-    self[:header_remote_url] = ''
+    self.avatar = nil
+    self.header = nil
+
     save!
   end