Remove :puma_phase metrics

What does this MR do?

We don't use phase restarts, as we use preload_app:
https://github.com/puma/puma/blob/master/README.md#clustered-mode
:puma_phase values will always be zero.

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

Related to #65278 (closed)