[glass] Don't check IsConnected for NT widgets (#4674)

Checking this isn't required, and prevented these widgets from working
in the simulation GUI without another NT client connected.

glass/src/libnt/native/cpp/NTDifferentialDrive.cpp

@@ -56,5 +56,5 @@ void NTDifferentialDriveModel::Update() {
 }
 
 bool NTDifferentialDriveModel::Exists() {
-  return m_inst.IsConnected() && m_lPercent.Exists();
+  return m_lPercent.Exists();
 }