I would definitely keep it. Why? Because each time Cloudflare needs to update your website’s cache from one of its multiple datacenters around the world, after a cache clear for example, it needs to make a roundtrip to your web server. If you don’t have a cache system on your server, these roundtrips will take much longer and will put more strain on your web server.
You also need to think about web crawlers and SEO. After a Cloudflare cache clear, a cached page will be generated after the first visit, but only for the visitor’s region. So if a Google crawler comes along and accesses the same page from a different region, Cloudflare’s datacenter from that region will access your web server again to pull the page. If you don’t have a cache system on your web server, that will be a lengthy trip for the crawler and might have an impact on your SEO score. The impact will most probably be minor, but you should always put the chances on your side.
In a perfect world, and I’m sure @yevgen would agree, Cloudflare would populate the cache from its own datacenters instead of making many lenghty roundtrips to web servers. So for example, Cloudflare could pull a web page from its New York datacenter to populate the cache in its London datacenter. This would mean less roundtrips to web servers and an almost instantaneous delivery of content all over the world. I would be curious to know if it’s something Cloudflare has already considered implementing. Maybe this is something covered by Argo?