refactor(combobox): remove unused interface (#8552)

**Related Issue:** N/A

## Summary

✨🧹✨

Author: jcfranco

packages/calcite-components/src/components/combobox/interfaces.ts

@@ -1,7 +1,2 @@
-export interface listItem {
-  label: string;
-  value: string;
-}
-
 export type ComboboxChildElement = HTMLCalciteComboboxItemElement | HTMLCalciteComboboxItemGroupElement;
 export type SelectionDisplay = "all" | "fit" | "single";