Use blank role for custom selectbox on macOS (microsoft#183460)

Acts as a workaround for and fixes microsoft#180115

Author: rzhao271

src/vs/base/browser/ui/selectBox/selectBoxCustom.ts

@@ -742,7 +742,7 @@ export class SelectBoxList extends Disposable implements ISelectBoxDelegate, ILi
 					return label;
 				},
 				getWidgetAriaLabel: () => localize({ key: 'selectBox', comment: ['Behave like native select dropdown element.'] }, "Select Box"),
-				getRole: () => 'option',
+				getRole: () => isMacintosh ? '' : 'option',
 				getWidgetRole: () => 'listbox'
 			}
 		});