Remove the in-memory cache (#6037)

Shields has long had an in-memory cache with some complex logic for determining when the cached values are used and when they are flushed. At the time this was implemented, squeezing cache performance was helpful since there was no downstream cache. For years now we've used Cloudflare as a CDN, so trying to cache onboard is less useful than before. Furthermore, since the cache is very small and only used in fairly restrictive ways, it probably has very little impact on performance.