Leverage service from DELETE /projects/:id/runners/:runner_id

What does this MR do and why?

We currently have inconsistent behavior around unassigning a runner from a project:

• DELETE /projects/:id/runners/:runner_id REST API says that "It works only if the project isn’t the only project associated with the specified runner"
• Admin::RunnerProjectsController#destroy and Projects::RunnerProjectsController#destroy do not apply any restrictions.

This MR harmonizes the behavior everywhere by leveraging the service in all places that unassign a runner from a project.

References

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• Ensure that sharding_key_id on project runner p... (#517114 - closed)

MR acceptance checklist

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Screenshots or screen recordings

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How to set up and validate locally

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

• Go to a project, such as http://gdk.test:3000/gitlab-org/gitlab-shell/-/settings/ci_cd#js-runners-settings

• Create runner

• Go to another project, such as http://gdk.test:3000/gitlab-org/gitlab-test/-/settings/ci_cd#js-runners-settings

• In Runners/Other available runners, locate the runner you created and click Enable for this project

• In the terminal, run the following command, replacing the :project_id and :runner_id by the respective numeric IDs:

  curl -v --request DELETE --header "PRIVATE-TOKEN: $GITLAB_GDK_TOKEN" "http://gdk.test:3000/api/v4/projects/:project_id/runners/:runner_id"

• Refresh the page at http://gdk.test:3000/gitlab-org/gitlab-test/-/settings/ci_cd#js-runners-settings and confirm that the runner now shows again as Enable for this project