Refactor DuoChat styles

What does this MR do and why?

Refactored DuoChat's CSS by moving removing a class that was used as a utility class but not meant to be one, to a dedicated class without side-effects.

Original Implementation plan: gitlab-ui!3996 (comment 1792730477)

The current implementation is working fine at the moment. But the class was never meant to be used as a utility so we are having some side-effects that could harm us in the future. This MR changes this.

EE: true

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sideeffects

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