Remove pql_three_cta_test

What does this MR do and why?

We want to remove this experiment completely.

Screenshots or screen recordings

group page	project page

How to set up and validate locally

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK

bin/rails c

Feature.disable(:showcase_free_security_features)

• Login as root. Visit group with projects.
• Visit group's Security page.
• Make sure there are no Contact sales button.
• Visit project's Security page.
• Make sure there are no Contact sales button.

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 #398655 (closed)