This partially reverts commit a1388592d8.
Fetching the authenticate service HPKE public key is required only for
the stateless authentication flow. Now that Pomerium will instead use
the older (stateful) authentication flow when configured for a
self-hosted authenticate service, this logic shouldn't be needed at all.
Removing this logic should also make it easier to test against a local
instance of the hosted authenticate service.
Fetch the HPKE public key only when configured to use the hosted
authenticate service. Determine whether we are using the hosted
authenticate service by comparing the resolved authenticate domain with
a hard-coded list of hosted authenticate domains.
Extract this list of hosted authenticate domains to the internal/urlutil
package in order to keep a single source of truth for this data.
