Add cleanup step to Grafana and Prometheus ignored files
Created by: daxmc99
This adds cleanup to the generated Grafana and Prometheus files located in docker-images/grafana/provisioning/dashboards/sourcegraph/ and docker-images/prometheus/config/
I found this when I realized my local dev environment was not being refreshed. This means that even though we were not generating a config it would still be visible due to not being cleaned up.
Fixes https://github.com/sourcegraph/sourcegraph/issues/12280