Data list - remove custom styles for width modifiers examples

[mcoker]

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With penta, the global vars that create these react-tokens that are used for custom styles will be removed, so we'll want to address that before penta is released (or whenever core removes the global vars and the react-tokens break).

The custom styles are used in the width modifiers example, but core has an equivalent example without custom styles, so I think we can just remove the custom stuff on the react side.

https://github.com/patternfly/patternfly-react/blob/3a88c11a91941fd01688e978a5c07b947f3a078a/packages/react-core/src/components/DataList/examples/DataList.md?plain=1#L26-L29

https://github.com/patternfly/patternfly-react/blob/3a88c11a91941fd01688e978a5c07b947f3a078a/packages/react-core/src/components/DataList/examples/DataListWidthModifiers.tsx#L23-L26

https://github.com/patternfly/patternfly-react/blob/3a88c11a91941fd01688e978a5c07b947f3a078a/packages/react-core/src/components/DataList/examples/DataListWidthModifiers.tsx#L49-L56

And anywhere previewPlaceholder is referenced