Revert "qemu: Check duplicate WWNs also for hotplugged disks"
Similarly to commit 5da28cc3069b573f54f0bcaf8eb75476bcfdc6e9 this check
actually does not make sense since duplicate WWNs are used e.g. when
multipathing disks.
This reverts commit 780fe4e4baf7e2f10f65ba1a34f9274fc547cad2.