refactor(combobox-item): drop unused variable (#7587)

**Related Issue:** N/A

## Summary

Dropping lingering, unused variable.

Author: jcfranco

packages/calcite-components/src/components/combobox-item/combobox-item.tsx

@@ -109,8 +109,6 @@ export class ComboboxItem implements ConditionalSlotComponent, InteractiveCompon
 
   @Element() el: HTMLCalciteComboboxItemElement;
 
-  isNested: boolean;
-
   // --------------------------------------------------------------------------
   //
   //  Lifecycle