Remove contextPerRenderKey option

[AgnesToulet]

contextPerRenderKey option was added to support taking a lot of dashboard screenshots at once to get dashboard thumbnails. It's a feature that was removed from Grafana.

As this option depends on a library we own (poolpeteer, a fork of puppeteer-cluster) and we don't want to keep the maintenance of this library for an option that we don't use, we need to remove this option from the image-renderer codebase.