Remove unused hide_dismissed_vulnerabilities feature flag code & yml file

ee/app/controllers/projects/security/dashboard_controller.rb

@@ -6,10 +6,6 @@ module Projects
       include SecurityDashboardsPermissions
 
       alias_method :vulnerable, :project
-
-      before_action only: [:index] do
-        push_frontend_feature_flag(:hide_dismissed_vulnerabilities)
-      end
     end
   end
 end

ee/config/feature_flags/development/hide_dismissed_vulnerabilities.yml

----
-name: hide_dismissed_vulnerabilities
-introduced_by_url: 
-rollout_issue_url: 
-group: 
-type: development
-default_enabled: false