[workers/http] Drop caching support

This was primarily useful for KIO-based browsers (i.e. Konqueror). It allows for fine-grained control about caching policies via a KCM. This KCM was dropped a while ago because it is no longer actually relevant for any kio-http user.

Current kio-http users mostly use it to do HTTP API calls, where caching is often undesired. For those cases where caching would be useful one typically needs more control from the application code than is exposed here.

Given the very limited usefulness of having caching here remove it.