Remove automatic cookie persistence in the HTTP worker

This is a leftover from times when this was mainly used for a web browser. For the remaining uses of this automatic cookie storage is generally not desired though, and therefore disabled explicitly already. Should applications want to persist cookies, that is still possible, but needs to be done explicitly now, same as with QNAM.

The bigger goal here is to eventually move the KCookieJar kded module and KCM to its only remaining users, Konqueror.