The migrate_data kwarg was added to the virt driver destroy()
method in change bc45c56f10 but
was never actually used, i.e. nothing passes migrate_data to
the destroy() method. The compute manager passes migrate_data to
the cleanup() method. This change just removes the unused kwarg
so it's not confusing for new virt drivers like powervm.
Change-Id: I6cca3b5ddd387dac86750be70f49c764a1be2fca