Remove `confidence` from vulnerability issue

What does this MR do and why?

Remove confidence from vulnerability issue

The confidence field was deprecated in 15.3 and removed in 16.0.

This removes its presence from issues created from a vulnerability.

This field always shows unknown from %16.0 as the data was removed then.

Related issue: #415406 (closed)

Removal announcement: !118546 (merged)

Screenshots or screen recordings

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

Before	After
See related issue

How to set up and validate locally

1. Populate a project with vulnerabilities (e.g. bundle exec rake "gitlab:seed:vulnerabilities[gitlab-org/gitlab-test]")
2. Open the vulnerability report (e.g. http://gdk.test:3000/gitlab-org/gitlab-test/-/security/vulnerability_report)
3. Click on any vulnerability
4. Click "create issue"
5. Create the issue
6. Observe that there's no confidence in the description of the created issue.

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 #415406 (closed)