Remove docs for never-implemented Destination service heartbeat
Fixes
@@ -21,11 +21,7 @@ import "common/common.proto";
// the service.
//
// The controller is expected to send an Update every time there is a
-// change in service discovery. The controller is also expected to send an
-// update at least once every ADDRESS_UPDATE_INTERVAL to indicate that the
-// controller is still healthy. If no service discovery updates have taken
-// place, the controller can simply send an empty `add`. The controller may
-// determine the value of ADDRESS_UPDATE_INTERVAL.
+// change in service discovery.
//
// The client MUST be prepared to receive messages in any order and the client
// MUST be able to cope with the presence or absence of redundant messages.
