fix(select): revert 2fed2c1 that delegates to list for single selection logic

PiperOrigin-RevId: 318345783

Author: allan-chen

packages/select/src/mwc-select-base.ts

@@ -578,8 +578,6 @@ export abstract class SelectBase extends FormElement {
       },
       addClassAtIndex: () => undefined,
       removeClassAtIndex: () => undefined,
-      getSelectedIndex: () => this.index,
-      setSelectedIndex: () => undefined,
       isTypeaheadInProgress: () =>
           typeahead.isTypingInProgress(this.typeaheadState),
       typeaheadMatchItem: (nextChar, startingIndex) => {