Remove "category" from known events

What does this MR do and why?

category field for known_events are obsolete and getting completely removed within this MR.

This is the second step on removing known_events completely.

Screenshots or screen recordings

All covered by UsageData and intrumentation classes specs

How to set up and validate locally

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

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 #387786 (closed)