Loadpoint UI: assign to real circuits only

[naltatis]

fixes #30380
replaces #30415

Remove the option to assign a loadpoint to the gridcontrol circuit directly.

🧹 also removes redundant unassigned and --- select options

[sourcery-ai]

Hey - I've left some high level feedback:

• The restriction that gridcontrol must not be user-assignable is now enforced directly in the UI filter; consider centralizing this rule in a shared model or API layer so the constraint is not duplicated or bypassed by other consumers of the circuits list.
• With circuitOptions now relying on the PropertyField placeholder for the empty option, double-check that all usages of this component provide a suitable placeholder so you don't end up with a select that has no way to clear the circuit assignment.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The restriction that `gridcontrol` must not be user-assignable is now enforced directly in the UI filter; consider centralizing this rule in a shared model or API layer so the constraint is not duplicated or bypassed by other consumers of the circuits list.
- With `circuitOptions` now relying on the `PropertyField` placeholder for the empty option, double-check that all usages of this component provide a suitable placeholder so you don't end up with a select that has no way to clear the circuit assignment.

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