Remove configuration UI for thumbnailer
The options are very undiscoverable in dolphin and having an option for this seems quite overkill to me. If people really want to have this kind of configurability they should be using an image viewer instead of looking at the thumbnails.
For compatibility the config file is still read.
It also causes issues from a dependency kind of view. Task: https://phabricator.kde.org/T14695
@alemariusnexus @meven Based on our previous discussion in !5 (merged) I hope this change is okay for you :)