[server] Cannot modify partition because of RAID

I'm installing 18.04 on a new machine and need to modify the partitioning of devices on which I installed software RAID. Per Want to Remove Software RAID (closed as solved) I have done this:
mdadm --stop /dev/md0
mdadm --stop /dev/md1
mdadm --zero-superblock /dev/sda1
mdadm --zero-superblock /dev/sda2
mdadm --zero-superblock /dev/sdb1
mdadm --zero-superblock /dev/sdb2

The installer no longer shows the RAID arrays but still sees the partitions as being RAID and won't allow them to be changed:

No modifications can be made to partition #1 of device SCSI (0,0,0) (sda) for the following reasons:
In use by software RAID device md0.

But /dev/md0 no longer exists.

How can I get back to where I can re-partition?