Stop setting AssumeReachable on chutney relays and bridges

We need to stop setting AssumeReachable on relays and bridges in chutney networks, so they do reachability self-tests.

We should continue to set AssumeReachable on authorities (including the bridge authority), because it disables authority to relay reachability tests. (These tests are still performed on a half-hourly schedule, even in chutney networks. And they are out of scope for Sponsor 55.)

After we implement this change, we should also be able to implement #33581 (moved).