Fix subsystem LiveWindow usage (#2202)

wpilibNewCommands/src/main/java/edu/wpi/first/wpilibj2/command/SubsystemBase.java

@@ -76,7 +76,6 @@ public abstract class SubsystemBase implements Subsystem, Sendable {
    */
   public void addChild(String name, Sendable child) {
     SendableRegistry.addLW(child, getSubsystem(), name);
-    SendableRegistry.addChild(this, child);
   }
 
   @Override

wpilibNewCommands/src/main/native/cpp/frc2/command/SubsystemBase.cpp

@@ -63,5 +63,4 @@ void SubsystemBase::SetSubsystem(const wpi::Twine& name) {
 void SubsystemBase::AddChild(std::string name, frc::Sendable* child) {
   auto& registry = frc::SendableRegistry::GetInstance();
   registry.AddLW(child, GetSubsystem(), name);
-  registry.AddChild(this, child);
 }