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.

packages/ctl/ctl.ts

@@ -38,7 +38,6 @@ class ControlConnector extends lib.WebSocketClientConnector {
 			new lib.MessageRegisterControl(
 				new lib.RegisterControlData(
 					this._token,
-					"clusterioctl",
 					version,
 				)
 			)