Remove product_category from event dictionary guide and generator

What does this MR do and why?

As product_category is being removed from events, this MR removes any mention of product_category from the docs.

This MR also removes product_category from the event generator so that future events will not be generated with a value that we will remove.

How to set up and validate locally

Run the generator as described here: https://docs.gitlab.com/ee/development/snowplow/event_dictionary_guide.html#create-a-new-event-definition

Verify that the generated file doesn't contain a product_category key.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #402989 (closed)