xapcmd: rely on File::Temp cleanup for temporary dir

xapcmd: rely on File::Temp cleanup for temporary dir

remove_tree from File::Path 2.09 (from Perl 5.16.3 on CentOS 7.x)
doesn't seem to work properly on File::Temp objects.   Since
File::Temp->newdir sets CLEANUP=>1 by default anyways, we'll
just rely on that to perform cleanup instead of doing it ourselves.