Remove agent string

The agent string was inspired by the HTTP User-Agent header to serve as
a way to identify what's connecting to the cluster.  While there may
have been vision of other software connecting to a controller than the
Clusterio provided ones this seems highly unlikely to ever happen.

Remove redundant agent string for now.

test/lib/command.js

@@ -16,7 +16,7 @@ describe("lib/command", function() {
 	let testControl = new mock.MockControl(controlConnector);
 	let testController = new lib.Link(controllerConnector);
 	testController.handle(
-		lib.HostListRequest, () => [new lib.HostDetails("test", "0.1", "Test Host", 11, false)]
+		lib.HostListRequest, () => [new lib.HostDetails("0.1", "Test Host", 11, false)]
 	);
 	testController.handle(
 		lib.InstanceDetailsListRequest, () => [new lib.InstanceDetails("Test Instance", 57, 4, undefined, "stopped")]