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

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.