After you merge a GitLab or GitHub pull request, you usually delete the topic branch in the remote repository to maintain repository hygiene. However, this action deletes the topic branch only in the remote repository. Your local Git repository also benefits from routine cleanup.To synchronize the information in your local repository with the remote repository, you can execute the git prune command to delete the local reference to a remote branch in your local repository.

Source: LXer – Delete the local reference to a remote branch in Git