Change CreateMergeRequestService rollback for sec

Change the rollback mechansim of
SecurityFinding::CreateMergeRequestService
to support a separate sec database.

EE: true
Changelog: changed

ee/app/services/vulnerabilities/security_finding/create_merge_request_service.rb

@@ -8,24 +8,17 @@ def execute
 
         @error_message = nil
 
-        merge_request = nil
-        Gitlab::Database::QueryAnalyzers::PreventCrossDatabaseModification.temporary_ignore_tables_in_transaction(
-          %w[
-            internal_ids
-            merge_requests
-            merge_request_user_mentions
-            vulnerability_merge_request_links
-          ], url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/480003'
-        ) do
-          merge_request = ApplicationRecord.transaction do
-            vulnerability = find_or_create_vulnerability
-            create_merge_request(vulnerability).tap do |merge_request|
-              create_vulnerability_merge_request_link(merge_request, vulnerability)
-            end
-          end
-        end
+        vulnerability = find_or_create_vulnerability
+        merge_request = create_merge_request(vulnerability)
+        create_vulnerability_merge_request_link(merge_request, vulnerability)
+
+        success_response(merge_request)
+
+      rescue ActiveRecord::Rollback
+        vulnerability.destroy if vulnerability.present?
+        merge_request.destroy if merge_request.present?
 
-        merge_request.present? ? success_response(merge_request) : error_response(@error_message)
+        error_response(@error_message)
       end
 
       private

ee/spec/services/vulnerabilities/security_finding/create_merge_request_service_spec.rb

@@ -150,7 +150,7 @@
         project.vulnerabilities.count
       }.by(0)
        .and(change(Vulnerabilities::MergeRequestLink, :count).by(0))
-       .and(change(Vulnerabilities::MergeRequestLink, :count).by(0))
+       .and(change(MergeRequest, :count).by(0))
     end
   end