Bring method back that was mistakenly removed

What does this MR do?

We recently had a regression where a method was mistakenly removed. The specs didn't catch. The bug is that the installation pod is not removed after installation is finished. This is not obvious to find since it does not raise any error, simply does not clean the kubernetes pod from the cluster.

Here is the commit that added the regression: https://gitlab.com/gitlab-org/gitlab-ce/commit/1cec47ec1a9ebba716a5396b560bf7efc3319020

This MR brings the method back for removing the pod and improves the specs to guard agains this happening again.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers