[ticket/11916] Remove files from hidden attach list after deletion

The hidden attachment_data list needs to be updated if a file got removed.
This was not done until now and caused a general error after submitting a
post if a file was removed after uploading it.

PHPBB3-11916
Merge remote-tracking branch 'Marc/ticket/11916' into develop

* Marc/ticket/11916:
  [ticket/11916] Remove files from hidden attach list after deletion