Storybook: Remove Emotion as JSX import source

[aduth]

What?

Updates Storybook configuration to remove the jsxImportSource override to @emotion/react.

Follow-up to #80426 and specifically #80426 (comment).

Why?

1. Fixes the issue described in Storybook: fix Emotion jsxImportSource #80426 (comment) where assigning jsxImportSource causes the import source to be injected into packages that don't necessarily define the dependency themselves, which will break under isolated npm install being explored in Explore using npm install-strategy=linked #75814.
2. It's not necessary: As I understand, the @emotion/react JSX import source is primarily necessary to support css prop assignment to components, but the last instance of this was removed in Components: migrate View away from Emotion #79443.

How?

Removes the explicit jsxImportSource override in the Storybook configuration so that it falls back to the default (react).

@emotion/react still needs to be a dependency in storybook/package.json because it is a peer dependency of @emotion/styled, which is still used in the @wordpress/storybook tool package.

Testing Instructions

Repeating Testing Instructions from #79443.

It's also worth testing that the live Storybook (via both npm run storybook:dev and npm run storybook:build build output) is unaffected, since this configuration is common to all Storybook builds:

1. Run npm run storybook:build && cd storybook/build && php -S localhost:8000
2. Go to http://localhost:8000
3. Observe that stories load as expected with no errors in browser console

Use of AI Tools

Used Cursor IDE + Auto (likely Composer) model to explore the initial proposal at #80426 (comment) and understand the viability of removing the override. Manually reviewed.

[github-actions]

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Co-authored-by: aduth <aduth@git.wordpress.org>
Co-authored-by: manzoorwanijk <manzoorwanijk@git.wordpress.org>

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[github-actions]

Size Change: 0 B

Total Size: 7.75 MB

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[manzoorwanijk]

Better. Thanks