Remove outdated convert instructions

See merge request https://gitlab.com/gitlab-org/grafana-dashboards/-/merge_requests/66



Merged-by: John Jarvis <jarv@gitlab.com>
Approved-by: Andrew Newdigate <andrew@gitlab.com>
Approved-by: John Jarvis <jarv@gitlab.com>
Co-authored-by: Katrin Leinweber <kleinweber@gitlab.com>