Automatically delete empty directories in CleanerChore

Currently the CleanerChore asks cleaner delegates if both directories and files should be deleted. However, this leads to somewhat odd behavior in some delegates - you don't actually care if the directory hierarchy is preserved, the files; this means you always will delete directories and then implement the logic you actually want for preserving files. Instead we can handle this logic one layer higher in the CleanerChore and let the delegates just worry about preserving files.