Remove the ability to create read-only disks

There is currently [an issue](https://github.com/oxidecomputer/propolis/issues/1059) that is making VMs backed by read-only disks unhappy (they do not boot). While RO disks is not an R18 feature, the team doesn't feel comfortable shipping a feature that doesn't work, so we'd like to remove it from the UI in 18 while the issue is investigated and resolved.