Fix nil pointer error from proxy registration check (#23021)

Author: pajay-rao

command/connect/envoy/envoy.go

@@ -528,11 +528,7 @@ func (c *cmd) proxyRegistration(svcForSidecar *api.AgentService) (*api.AgentServ
 		// fallback to localhost as the gateway has to reside in the same network namespace
 		// as the agent
 		tcpCheckAddr = "127.0.0.1"
-		ds, err := netutil.IsDualStack(nil, false)
-		if err == nil {
-			return nil, err
-		}
-		if ds {
+		if c.useIPv6loopback {
 			tcpCheckAddr = "::1"
 		}
 	}