TeX equations in Markdown document generate small scrollbars

What does this MR do and why?

When fixing #394770 (closed), using overflow: auto on inline math accidentally introduced unnecessary scrollbars.

Use instead overflow: hidden to no longer show scrollbars.

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Screenshots or screen recordings

Before	After

How to set up and validate locally

If you're running on a Mac, make sure "Show scrollbars" is set to "Always" in "System Settings -> Appearance"

1. Running on master, add the markdown For any value $x_0$. to a comment. You should see the scrollbars.
2. Running on this MR, do the same. The scrollbars should not be visible.

Related to #429512 (closed)