Capture running builds on non-shared runners (remove feature flags)

What does this MR do and why?

:rotating_light::rotating_light: This MR should not be merged before April 23rd, to allow building up confidence in the functionality :rotating_light::rotating_light:

This MR removes the add_all_ci_running_builds and remove_all_ci_running_builds feature flags (rollout issue) after a successful rollout of !147943 (merged) completed at 2024-04-19 13:01:31 UTC.

Changelog: changed

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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n/a

How to set up and validate locally

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

1. Select a project in your GDK.

2. Create and register a project runner in that project with the tags: shell, gdk.

3. Edit the .gitlab-ci.yml to include the following content:

   default:
     tags:
       - shell
       - gdk
   
   build1:
     stage: build
     script:
       - echo "Do your build here"
       - sleep 300

4. Commit and ensure a pipeline is run.

5. On the GDK Rails console, confirm that the Ci::RunningBuild was created:

   > Ci::RunningBuild.where.not(runner_type: :instance_type).exists?
     Ci::RunningBuild Exists? (0.4ms)  SELECT 1 AS one FROM "ci_running_builds" WHERE "ci_running_builds"."runner_type" != 1 LIMIT 1 /*application:console,db_config_name:ci,console_hostname:gitlab-macbookpro.work.pombei.ro,console_username:pedropombeiro,line:(pry):12:in `__pry__'*/
   => true