Is there any way to remove the partition key, or do we simply need to create a new table with the structure we need, drop the old one, then rename the new one to that original name and reattach the partitions?
I'm not super familiar with partitioning but...you can probably do it in the reverse order and avoid the rename. A partitioned table without partitions seems useless so just drop it, then create a new one with the same name with the correct defintion.
The documentation is indeed clear though you may be looking in the wrong spot: