Delete prometheus panel group model and specs

What does this MR do and why?

Deletes PrometheusPanelGroup model and its specs.

There was no usage in other files except for specs.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #397142