Remove generate_cube_query flag and feature

What does this MR do and why?

• Removes generate_cube_query feature flag, feature, tests and documentation.

References

Screenshots or screen recordings

No changes

Page	Screenshot
Security dashboard
Group analytics dashboards list
Group analytics dashboard example
Project analytics dashboards list
Project analytics dashboard example

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.