Remove secure_vulnerability_training feature flag

What does this MR do and why?

This MR removes the feature flag secure_vulnerability_training

Changelog: changed

Related to #346308 (closed)

Screenshots or screen recordings

Config	Training URL

How to set up and validate locally

1. Enable security training for vulnerabilities > https://docs.gitlab.com/ee/user/application_security/vulnerabilities/#enable-security-training-for-vulnerabilities
2. View security training for a vulnerability > https://docs.gitlab.com/ee/user/application_security/vulnerabilities/#enable-security-training-for-vulnerabilities

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.