Fix some warnings in the FormAutocomplete component

[t-wright]

Description

We're currently passing the classNames prop to <FormSelectArrow /> which gets passed on to the icon itself, resulting in the warning: React does not recognize the classNames prop on a DOM element

We also currently expect a string for the label prop, which causes a prop type warning when we pass it an element (e.g. by including screen reader text within the label).

Steps to Test

1. Pull down PR.
2. npm run dev.
3. Open Storybook.
4. Check the console, verify no React does not recognize the classNames prop on a DOM element warning is present.
5. Update src/system/NewForm/FormAutocomplete.stories.jsx, passing <></> as the label to <FormAutocomplete />
6. Verify no prop type warnings are present in the console.

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