Remove unused vulnerabilities-row styles

What does this MR do and why?

It removes some unused CSS from a page specific stylesheet.

The affected stylesheet ee/app/assets/stylesheets/page_bundles/security_dashboard.scss gets included in the following pages:

• Project security dashboard - eg.: http://localhost:3000/h5bp/html5-boilerplate/-/security/dashboard
• Project vulnerability report - eg.: http://localhost:3000/h5bp/html5-boilerplate/-/security/vulnerabilities
• Group security dashboard - eg.: http://localhost:3000/groups/h5bp/-/security/dashboard
• Group vulnerability report - eg.: http://localhost:3000/groups/h5bp/-/security/vulnerabilities
• Instance security dashboard - eg.: http://localhost:3000/-/security/dashboard
• Instance vulnerability report - eg.: http://localhost:3000/-/security/vulnerabilities

It appears that none of those areas include any elements with the .vulnerabilities-row class.

The only place where it is used is the pipeline security tab, but the stylesheet is not included there.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Check out the pages listed above and make sure that there are no visible changes
2. Check out the pipeline security tab and make sure that there are no visible changes

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 #367815 (closed)