[mdns-avahi] handle potential unknown Avahi states gracefully

This change avoids `assert(false)` on `AvahiEntryGroupState` and
`AvahiClientState` `switch` statements. This prevents potential
crashes if Avahi introduces new states in future versions.

src/mdns/mdns_avahi.cpp

@@ -570,10 +570,6 @@ void PublisherAvahi::HandleGroupState(AvahiEntryGroup *aGroup, AvahiEntryGroupSt
  case AVAHI_ENTRY_GROUP_UNCOMMITED:
  case AVAHI_ENTRY_GROUP_REGISTERING:
  break;
-
- default:
- assert(false);
- break;
  }
 }
 
@@ -696,10 +692,6 @@ void PublisherAvahi::HandleClientState(AvahiClient *aClient, AvahiClientState aS
  case AVAHI_CLIENT_CONNECTING:
  otbrLogInfo("Avahi client is connecting to the server");
  break;
-
- default:
- assert(false);
- break;
  }
 }