Clear runner heartbeat when the last manager is removed

What does this MR do and why?

Address Change runner "configuration" status to offline... (#422066 - closed)

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Screenshots or screen recordings

After removing the last runner manager:

Before	After

Note that we now have this never contacted status. We need Introduce runner creation status (#436475 - closed) to address it.

How to set up and validate locally

1. Create and register the runner
2. Unregister it
3. Refresh the runner page