Remove Default and Plain Styles from Quote Block

[karthick-murugan]

What?

Fixes #49776

This PR removes the "Plain" style from the core Quote block by eliminating it from the block.json file.

Why?

The styles for "Default" and "Plain" did not offer significant visual differentiation, causing unnecessary complexity. Removing these ensures a simpler, more consistent block styling that aligns with the direction of block-based themes, which avoid relying on bundled styles.

How?

The block editor block styles definitions for both "Default" and "Plain" variations were removed from the Quote block's styles component in the block.json file. This simplifies the block's styling and improves compatibility with modern block themes.

Testing Instructions

1. Open the WordPress editor (post or page).
2. Insert a Quote block.
3. Verify that the "Default" and "Plain" style variations are no longer available.
4. Observe that the block renders consistently across different themes, regardless of their support for wp-block-styles.

Screenshots or screencast

Before

After

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Co-authored-by: karthick-murugan <[email protected]>
Co-authored-by: carolinan <[email protected]>
Co-authored-by: richtabor <[email protected]>

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