Unfortunately there is no way to remove the cloud tier bucket without destroying the aggregate. The exception is a fabric pool mirror where you can remove one, but must have at least one once added. Attaching a tier is permanent.
Workarounds possibly for you..set the tiering policy to none for all volumes so all data is in the performance tier. The capacity tier will still be attached but not used. Or if enough spare drives, volume move the volumes from the FabricPool aggr to a non-FabricPool aggregate, then when empty, destroy the fabric pool aggregate and reclaim the disks.