core/CMakeLists: Make all web_service-related libraries private

Now that all external dependencies are hidden, we can remove
json-headers from the publically linked libraries, as the use of this
library is now completely hidden from external users of the web_service
library. We can also make the web_services library private as well,
considering it's not a requirement. If a library needs to link in
web_service, it should be done explicitly -- not via indirect linking.