Fix a performance/memory issue in lib/api/services.rb

What does this MR do?

This fixes a performance/memory issue in lib/api/services.rb. This reduces the number of retained objects and memory, as explained in https://gitlab.com/gitlab-org/gitlab-ce/issues/43620.

Are there points in the code the reviewer needs to double check?

I don't think there are.

Why was this MR needed?

Because I stumbled upon this, and because performance matters!

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43620 (closed)