Remove frontend EM from runner bug triage process

It's just creating noise to ping the FE EM for Runner issues. In the rare case a frontend bug comes in the backend EM can inform the frontend EM.

When creating or changing a new triage policy or triage package:

• Consider adding documentation to the handbook pages for Triage Operations: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/source/handbook/engineering/quality/triage-operations/index.html.md
• Consider announcing to the appropriate Slack channels or week-in-review