Clean up Endpoint / BaseEndpointInfo a bit

Remove NodeName, which was unused because we only care about IsLocal
which was tracked separately.

Remove Zone, which was unused because it's from the old topology
system?

Fix up some comments which still referred to Endpoints vs
EndpointSlice differences.

Also remove an unhelpful helper function in endpoints_test.go

pkg/proxy/endpointslicecache.go

@@ -294,19 +294,9 @@ func (cache *EndpointSliceCache) addEndpoints(svcPortName *ServicePortName, port
 			continue
 		}
 
-		isLocal := false
-		nodeName := ""
-		if endpoint.NodeName != nil {
-			isLocal = cache.isLocal(*endpoint.NodeName)
-			nodeName = *endpoint.NodeName
-		}
+		isLocal := endpoint.NodeName != nil && cache.isLocal(*endpoint.NodeName)
 
-		zone := ""
-		if endpoint.Zone != nil {
-			zone = *endpoint.Zone
-		}
-
-		endpointInfo := newBaseEndpointInfo(endpoint.Addresses[0], nodeName, zone, portNum, isLocal,
+		endpointInfo := newBaseEndpointInfo(endpoint.Addresses[0], portNum, isLocal,
 			endpoint.Ready, endpoint.Serving, endpoint.Terminating, endpoint.ZoneHints)
 
 		// This logic ensures we're deduplicating potential overlapping endpoints