Remove Beta and Experiment from developer documentation

What does this MR do and why?

This MR removes Beta and Experiment from this documentation.

Previously we had a setting that was for all Beta and Experiment features. We've decided to make the toggle only control GitLab Duo Beta and Experiment features to resolve UI confusion. Enablement of GitLab Duo Beta/Experiment features already covered in the Duo documentation.

It's important to remove Beta and Experiment from this documentation because we need to make it clear that this setting is no long for ALL experiment and beta features.

Image	Description
	Remove this section, because we need to make it more clear that this toggle is only for Duo features
	The UI change

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