Removes factories on Cluster background migration

What does this MR do?

Removes factories on Cluster background migration: spec/lib/gitlab/background_migration/populate_cluster_kubernetes_namespace_table_spec.rb

According to our documentation (see spec/migrations/readme.md), we should use table helper on background migrations, to create a temporary ActiveRecord::Base derived model

This MR is needed for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24368

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer