Remove fallback to deployment_platform_cluster in `DeploymentMetrics`

What does this MR do?

Removes potentially incorrect, and slow fallback. #deployment_platform is relatively expensive and calling this after the fact means that this may not be the cluster that was deployed to.

https://gitlab.com/gitlab-org/gitlab-ce/issues/64105

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

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team