Offboards a compute VNet to be NSX unmanaged from a transit VNet.
All the VMs in the compute VNet need to be untagged before offboarding
the compute VNet. Offboarding status can be obtained from
/csm/azure/compute-vnets//status API. Upon successful offboarding
compute VNet, the onboard_step will be OFFBOARD_SUCCESSFUL
and op_status will be NOT_AVAILABLE. If any error is encountered
during offboarding, corresponding error_code and error_message
will be populated.