fix: Ruleset disablement of secondary identifier should filter primaries

What does this MR do?

Previously, disabling a primary identifier would filter scan.PrimaryIdentifiers but disabling a non-primary identifier would not properly filter the PrimaryIdentifier collection.

This also fixes a previous assertion removed in gitlab-org/gitlab#398809 (closed)

3 Minor cleanup tasks:

• Remove unneeded configuration from sast-ruleset-disable-ids.toml
• Switch from custom SetFeatureEnv func to t.Setenv
• Add missing names to TestDisableIdentifierReports testcases

What are the relevant issue numbers?

Follow-up to gitlab-org/gitlab#398809 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Ensure the report version matches the equivalent schema version
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer