client.shared.autotemp: Fix autotemp.tempdir.clean()

On NFS filesystems, when trying to remove the temporary
directory, we might end up with the same problem that
commit 5d1b5ba fixed (directory removal on NFS might
fail due to tmp files created by the NFS implementation).

So let's use utils.safe_rmdir once again to solve the
problem.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #771 from autotest/autotemp-fix

client.shared.autotemp: Fix autotemp.tempdir.clean()