Revert new default value of isClearable prop (#484)

Suddenly defaulting isClearable to true broke several Selects in ChMS
(and likely elsewhere) since they'd written onChange handlers had never
needed to expect null values before.

components/text-input/select.jsx

@@ -290,7 +290,7 @@ export function useCommonSelectProps(props, ref) {
 		inputValue,
 		defaultInputValue,
 		onInputChange,
-		isClearable = true,
+		isClearable,
 		isMulti,
 		...otherProps
 	} = props;