Remove feature flag ci_use_downstream_pipeline_duration_for_calculation

What does this MR do and why?

As titled, remove the feature flag ci_use_downstream_pipeline_duration_for_calculation for the fix of !109445 (merged)

Screenshots or screen recordings

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How to set up and validate locally

1. Enable the feature flag:

   rails runner 'Feature.enable(:ci_use_downstream_pipeline_duration_for_calculation)'

2. Create a project 356759-test-duration

3. Create .gitlab-ci.yml with the following content:

   # .gitlab-ci.yml
   run-child-pipeline:
     trigger:
       include: ci/child-pipeline.yml
       strategy: depend
   
   # ci/child-pipeline.yml
   stages:
     - build
     - test
   
   child-job:
     stage: build
     script:
       - echo "Running child job..."
       - sleep 20
       - echo "Running complete."
   
   run-grand-child-pipeline:
     stage: test
     trigger:
       include: ci/grand-child-pipeline.yml
       strategy: depend
   
   # ci/grand-child-pipeline.yml
   grand-child-job:
     script:
       - echo "Running grand child job..."
       - sleep 15
       - echo "Running complete."

4. Check if the pipeline takes the sum duration of child-job and grand-child-job

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #356759 (closed), #387942 (closed)