Remove cssKey param from CommonProps.cx (#45156)
Remove cssKey param from CommonProps.cx (#45156)

cx is a classNames with the first parameter bound.

cssKey param was removed from classNames here: https://github.com/JedWatson/react-select/commit/a860de4f9904473886cb66e7bfeea8539ca6a50f#diff-b194f39baaee25481bb4ebca83fd3850L44

`cx` now only takes 2 params as seen in usages through react-select
https://github.com/JedWatson/react-select/blob/master/packages/react-select/src/components/Group.js#L40

It's also not possible for classNames to return void.
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