[DevTools] Don't keep stale root instances we never mounted around#34504

eps1lon merged 2 commits intofacebook:mainfacebook/react:mainfrom

eps1lon:sebbie/09-16-_devtools_use_alternate_as_previous_fiber_when_reconciling_rooteps1lon/react:sebbie/09-16-_devtools_use_alternate_as_previous_fiber_when_reconciling_rootCopy head branch name to clipboard