itimpi,
I couldn't find an option to "unassign" the 2 cache drives, but I did find and select a "no" option which seemed to remove the drives and also give me 2 red warning boxes that the cache drives were missing. However, to the right of the greyed out start button, there was a message, which, if checked, said the cache drives would be removed and the array reconfigured without the cache drives. I clicked the box with a "check" mark and the array restarted with the two red warning boxes gone.
Here is a screenshot of my system after removing the 2 cache drives and restarting the array.
x-special/nautilus-clipboard
Can I now power down the array and remove the 2 drives and then restart the array without any problems?