Accordion Header: Fix RTL toggle button layout

[im3dabasia]

What?

Closes #71478

Removes unnecessary flex-direction CSS rules for RTL languages in the Accordion Header toggle button.

Why?

Flexbox automatically handles RTL positioning. The manual flex-direction overrides cause incorrect text/icon layout in RTL languages.

How?

Removes RTL CSS rules in packages/block-library/src/accordion/style.scss.

Testing Instructions

1. Switch to RTL language (Arabic/Hebrew)
2. Insert Accordion block
3. Verify toggle button text and icon position correctly
4. Test both default and icon-position-left variants

Screencast

[github-actions]

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[t-hamano]

Thanks for the quick fix!

P.S. I'm sure you can add labels to issues or PR, so feel free to add appropriate labels when you submit a PR. Also, feel free to merge your approved PRs 👍