ALTS: Do not pass a host to grpc_channel_create_pollset_set_call(); let the channel supply it. (#31867)

When the handshaker_service_url is in "host:port" format such as it
normally is when using ALTS in GCE (in which case it comes from then
this makes no difference as the authority and the URL are the same. But
when different URLs are used, the correct authority to use is not always
the same as the URL. For example if the URL is unix:///some/path then
the correct authority is "localhost". This is correctly computed by
grpc_core::UnixResolverFactory and stored as the channel's default
authority, but we throw that away when we override the authority for
individual RPCs.

Note indeed that the majority of other callers of grpc_channel_create_*
pass nullptr for the host/authority argument.

src/core/tsi/alts/handshaker/alts_handshaker_client.cc

@@ -733,16 +733,14 @@ alts_handshaker_client* alts_grpc_handshaker_client_create(
   client->handshake_status_details = grpc_empty_slice();
   client->max_frame_size = max_frame_size;
   client->error = error;
-  grpc_slice slice = grpc_slice_from_copied_string(handshaker_service_url);
   client->call =
       strcmp(handshaker_service_url, ALTS_HANDSHAKER_SERVICE_URL_FOR_TESTING) ==
               0
           ? nullptr
           : grpc_channel_create_pollset_set_call(
                 channel, nullptr, GRPC_PROPAGATE_DEFAULTS, interested_parties,
-                grpc_slice_from_static_string(ALTS_SERVICE_METHOD), &slice,
+                grpc_slice_from_static_string(ALTS_SERVICE_METHOD), nullptr,
                 grpc_core::Timestamp::InfFuture(), nullptr);
-  grpc_core::CSliceUnref(slice);
   GRPC_CLOSURE_INIT(&client->on_handshaker_service_resp_recv, grpc_cb, client,
                     grpc_schedule_on_exec_ctx);
   GRPC_CLOSURE_INIT(&client->on_status_received, on_status_received, client,