Kill the dete preview watcher

This code had the potential to time out. If a huge folder with pictures
for example was deleted then this could easily grow the number of files
to clean with a factor 5 (or more).

Now the previews just get cleaned up in the background. Which is good
enough for the 99% case

As a bonus this now also keeps the previews when in the trashbin so you
don't have a spiking server load when a user opens the trashbin view.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>