Simplify scanner logic for security reports

What does this MR do and why?

Simplify scanner logic for security reports related to scan finding rules because of the following:

1. Scanners are being already considered as part of security reports.
2. Removing any chance of ambiguity between persisted data versus yaml policy file.

How to set up and validate locally

1. Create a security project as described in the docs
2. Create a new scan result policy with the project owner (set scanners to anything but container scanning)
3. Push a new MR with gitlab-ci.yml the following changes:

include:
  - template: Security/Container-Scanning.gitlab-ci.yml
variables:
  DOCKER_IMAGE: python:3.4-alpine

4. Expected The security approval rule shouldn't require approval.

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.

ee/app/models/concerns/security/scan_result_policy.rb

@@ -25,15 +25,6 @@ def active_scan_result_policies
       def scan_result_policies
         policy_by_type(:scan_result_policy)
       end
-
-      def uniq_scanners
-        return [] if project.blank?
-
-        distinct_scanners = approval_rules.distinct_scanners
-        return [] if distinct_scanners.none?
-
-        distinct_scanners.pluck(:scanners).flatten.uniq
-      end
     end
   end
 end