Remove product_category from metrics related codebase

What does this MR do and why?

We discovered that product_category is not used right now anyway (see #393366 (comment 1295122962), #393366 (comment 1295377452)). Adding the product_category is an effort every time a metric is created.

This MR is the second step after !116065 (merged) where we removed product_category from metric related documentation.

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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 #393366 (closed)