Fix

Closes: #3466

The cleanup logic was moved out of the remove() method and into the cell property descriptor, where it properly belongs.

Previously, FixedAgent.remove() had to bypass the public cell API and modify private state directly. This created fragile behavior and inconsistent removal logic.

With this fix:

• The property descriptor now correctly handles cleanup.
• FixedAgent no longer needs to manually modify private attributes.
• Clearing an agent’s cell works consistently through the public API.

Without this change, any attempt to manually clear a FixedAgent’s cell outside of the remove() method would still trigger ValueError crashes.