Remove pipeline_name_in_api feature flag

What does this MR do and why?

Remove the feature flag for adding pipeline name to the pipelines API response.

The FF was enabled in prod on May 29th.

#398131 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Choose a project in your local GDK where you can run a CI pipeline.

2. Modify the .gitlab-ci.yml file to add a workflow:name. https://docs.gitlab.com/ee/ci/yaml/index.html#workflowname
   For example:

   workflow:
     name: "Test pipeline"

3. Start a pipeline.

4. Call the pipelines API: http://gdk.test:3000/api/v4/projects/root/flightjs/pipelines

5. The response should contain a pipeline name.

MR acceptance checklist

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