remove "indirect" vdev from boot pool

[jjb2016]

Hello - I have an arch linux system which boots into a root filesystem on ZFS 2.1.12. The root filesystem is in a pool with a single 2-way mirror vdev which previous had 2 x 12GB SSDs in it. I tried to upgrade the pool with two new nvme drives by adding a second 2-way mirror vdev with the two new drives and then removing the original mirror vdev with the two old drives - this appeared to work at first but I've discovered that it had left a "indirect" vdev in the pool. This is not something that I'm familiar with. Can it be removed? The problem I'm having is that if I physically remove the two old SSDs from the system then the system won't boot, presumably because the two removed drives are somehow still associated, and required, by these "indirect" vdev in the pool. How can remove the indirect vdev so that I can physically remove the old SSDs?