Replace deleteAll call with unlink call

The method deleteAll() doesn't officially exist on the Storage class as
it's not defined on the interface, which means it fails on the Quota
storage wrapper and might fail on some external storage classes.
Also, this here is the only use case for that one method.
Removed unused deleteAll method on Common storage class

The "deleteAll" method on the Common storage class isn't used anywhere.
Also, it isn't defined on the Storage interface so this fix removes it
completely.
Merge pull request #7371 from owncloud/core-storagemovedeleteall

Replace deleteAll call with unlink call on rename