Note attachment is deprecated and should be removed

Per https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3867#note_54972668

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/note.rb#L91 is deprecated. It should be possible to remove it, "as uploads now use the markdown uploader".

/cc @mbergeron @smcgivern