The short answer: You cannot change this currently.
The long answer: The dialog was added as a safe-guard because there were cases where you could lose all your files unintentionally, e.g. if your admin re-created your account and left it empty. The client would assume the files had gone and would replicate this (it could not know better), so it would replicate the data removal locally. The code is still there today just to be safe.
If you are fearless, you can patch Folder::slotAboutToRemoveAllFiles()
. Alternatively, you could open a bug report so we can solve this for everyone. What is your motivation to be able to do this without a prompt?
PS: The sources can be found on GitHub. URL and build instructions at http://doc.owncloud.org/desktop/1.5/building.html.