Undo nesting of security scanning

Remove navigation item from global navigation

• Link to the merge request that introduced the new page:
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Before: The following page does not appear in the global navigation:

• https://docs.gitlab.com/ee/user/application_security/dast/browser/configuration/requirements.html

After: The following page appears in the global navigation:

• https://rd-undo-nesting-of-security-scanning.docs.gitlab-review.app/ee/user/application_security/dast/browser/configuration/requirements.html

Additional details:

In Add docs nav entry for new page - Security scan... (!5277 - merged) I added an entry for a new "parent" page and nested entries under that page. This MR essentially undoes that MR because the nesting was too deep and caused other pages to be dropped from the docs global navigation.

This MR both undoes the nesting of some global navigation entries and removes the entry for the "Security scanning" page. I'm deleting that entry because the page is to be deleted in Reorganize security scanning feature docs (gitlab!176830 - merged).