Remove unused BackendManager.CreateFolder method.
Remove unused BackendManager.CreateFolder method.

This was introduced in revision aca4ed95cc7602711568b3563cdde80e8a621333
but without any apparent usages.

duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati