Remove product_category from metrics yml

Problem

When setting up a new metric the documentation suggests to add a product_category from the categories.yml.

These categories, however do not map to very general metrics, such as hostname, uuid or [license_billable_users](https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/config/metrics/license/20210531204603_license_billable_users.yml). Currently, these metrics use a product_category called collection, which is not part of categories.yml.

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

Desired Outcome

product_category is removed from all metrics files and documentation.