Revert "backup tools bup, storeBackup: implement sanity check" on behalf of Marc.

This reverts commit c823bc37d6b50dcd4e183963922da829a193f0d7. The
newly added test case needs more work to succeed on Hydra. See
<http://permalink.gmane.org/gmane.linux.distributions.nixos/11382>
for further details.