Fixed FTP and SMB to use rmdir() when deleting folders

Some storages need to use different calls for deleting files or folders,
usually unlink() and rmdir().

Fixes #4532 (SMB dir deletion)
Fixes #5941 (FTP dir deletion)

Note that the extra is_dir() should be fast because it's read from the
stat cache.
Merge pull request #6123 from owncloud/extstorage-deletedirs2

Fixed FTP and SMB to use rmdir() when deleting folders