Remove last_reviewed_on/review_in fields

Whilst this change happened on 29th October 2020, the commit date has been
backdated so that the 'last_updated' behaviour is unaffected. We don't want
to give a misleading impression that this document is more up to date than
it is because of the side-effect of removing some of its frontmatter.

source/manual/add-a-pingdom-check.html.md

@@ -4,8 +4,6 @@ title: Add a Pingdom check
 section: Monitoring
 layout: manual_layout
 parent: "/manual.html"
-last_reviewed_on: 2020-06-08
-review_in: 12 months
 ---
 
 GOV.UK uses [Pingdom](https://www.pingdom.com/) to provide an external view of