delete all children's previews when deleting a folder

please review @PVince81 @karlitschek 
fixes https://github.com/owncloud/core/issues/10484
I know, it's not really part of this PR, however, it would be really cool if you could add a little bit more explanation about the return value.

As far I can see it only returns `false` - this is very confusing for devs that are not familiar with this code.
(i.e. "Does a false mean that everything has been deleted?" or "Does it mean it failed?" etc...)