Remove product_category from metrics documentation

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.

Snowplow events out of scope of this MR

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MR acceptance checklist

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Related to #393366 (closed)