Bugfix: Av. offline files are not removed when "Local only" option is clicked

"Local only" option in remove dialog will be displayed when the selected folder contains at least one downloaded file, ignoring those available offline.
If the "Local only" option is displayed and clicked, available offline files will not be deleted.

https://github.com/owncloud/android/issues/4353
https://github.com/owncloud/android/pull/4399
