prop224: Fix one last instance of get_time_period_num() with now.

When purging last HSDir requests, we used time(NULL) for computing the
service blinded key but in all other places in our codebase we actually
use the consensus times. That can cause wrong behavior if the consensus
is in a different time period than time(NULL).

This commit is required for proper purging of HSDir requests.

src/or/hs_client.c

@@ -100,7 +100,7 @@ purge_hid_serv_request(const ed25519_public_key_t *identity_pk)
    * from the previous time period. That is fine because they will expire at
    * some point and we don't care about those anymore. */
   hs_build_blinded_pubkey(identity_pk, NULL, 0,
-                          hs_get_time_period_num(approx_time()), &blinded_pk);
+                          hs_get_time_period_num(0), &blinded_pk);
   if (BUG(ed25519_public_to_base64(base64_blinded_pk, &blinded_pk) < 0)) {
     return;
   }