Do you have multiple datastores? If so, perform a storage vMotion of the VM. This will consolidate the parent/delta disks into a single vmdk and make the disk expandable.
If not, you may have to shutdown the VM, clone it, and create a new manual pool from the VM. The clone should also have a single expandable vmdk. You -might- be able to remove the old linked clone and replace it with the full clone to prevent the need for separate pool, but I would need to do more research if that’s possible.
This is one of the reasons persistent linked clones are no fun!