This tried to assert that we never create a DiskCache object when private
browsing is turned on. However, when initializing, we still create a global
DiskCache, so this will hit when qutebrowser is started with private browsing
turned on via the config.
We could just not create the DiskCache at all when started in private browsing
mode, however we might still need it later when opening a non-private window.
766a1ebb6d