cmVSSetupHelper: Drop unused InstanceId field

Minimize the amount of information needed about a VS instance.

Source/cmVSSetupHelper.cxx

@@ -152,13 +152,6 @@ bool cmVSSetupAPIHelper::GetVSInstanceInfo(
   if (pInstance == NULL)
     return false;
 
-  SmartBSTR bstrId;
-  if (SUCCEEDED(pInstance->GetInstanceId(&bstrId))) {
-    vsInstanceInfo.InstanceId = std::wstring(bstrId);
-  } else {
-    return false;
-  }
-
   InstanceState state;
   if (FAILED(pInstance->GetState(&state))) {
     return false;

Source/cmVSSetupHelper.h

@@ -84,7 +84,6 @@ private:
 
 struct VSInstanceInfo
 {
-  std::wstring InstanceId;
   std::wstring VSInstallLocation;
   std::wstring Version;
   std::string VCToolsetVersion;