Cleanup timing Makefile

What does this merge request do and why?

Simplify code in support/makefiles/Makefile.timing.mk. I haven't found a way to get rid of make call completely, but this should be more readable already.

How to set up and validate locally

1. Ensure GitLab Docs is enabled by adding the below configuration in gdk.yml:

---
gitlab_docs:
  enabled: true

1. Check out to this merge request's branch.
2. Run gdk reconfigure to check if regenerating all configuration is successful.
3. Ensure tmp/gdk_update_timing.yml does not exist.
4. Run make gitlab-docs-update-timed
5. Run cat tmp/gdk_update_timing.yml

---
services:
  gitlab-docs-update:
    start: 1688144051
    end: 1688144096

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.