Remove explain_vulnerability feature flag

What does this MR do and why?

Release Explain this Vulnerability Button feature

Remove explain_vulnerability feature flag

Issue: [GA] Explain this Vulnerability: Remove explain... (#427350 - closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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 #407282