Remove dast_site_validation_drawer feature flag

Related to #390775 (closed)\

What does this MR do and why?

Removes dast_site_validation_drawer feature flag as it seems stable and working as expected.

Screenshots or screen recordings

No changes

How to set up and validate locally

1. Visit http://gdk.test:3000/flightjs/Flight/-/security/configuration/dast\
2. Create a site profile\
3. Expect the validate button is visible by default for new profiles.

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.