Remove outdated reference to prometheus collector on featured plugins

[asachs01]

          Noting here that there's a companion page in https://docs.sensu.io/sensu-go/latest/plugins/featured-integrations/prometheus/ that mentions the prometheus collector. We should eliminate the collector as part of the featured integrations page, given that it's pretty well deprecated.

Originally posted by @asachs01 in https://github.com/sensu/sensu-docs/issues/4168#issuecomment-1490747824