Tails Cloner: Remove one of the 30 second sleeps during partitioning

There is a 30 second sleep in the rescan_block_device() method which was added to avoid race conditions. That method is called three times in partition_device() (twice via get_system_partition()), resulting in a huge 90 second sleep.

This commit removes one of these sleeps, which doesn't seem to serve any purpose, because rescan_block_device() is called again immediately after.

refs #20131