fix: remove height in CSS to make avatar resizable

[pgerlach]

In the page "Learn" / "JavaScript in JSX with Curly Braces" in the last challenge, we're told to check if our fix worked by "changing the value of imageSize to 'b'. The image should resize after your edit.". However, the height of the avatar is fixed to 90px in the CSS, so changing the size of the base image doesn't make any visual difference.

I removed the height CSS attribute so that the exemple works as intended.

[github-actions[bot]]

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