Fix the Storybook production build

[AlanGreene]

Changes

For some reason @carbon/charts-react/styles.css isn't bundled correctly in the production Storybook build, even though the entry in the package's package.json point to the same location as @carbon/charts-react/styles. Drop the extension so the production build loads as expected.

The dev build was unaffected and continues to load the same content.

/kind misc

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Release Notes

NONE

[LyndseyBu]

/lgtm

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