AddTaskNode is adding task to cost_model, but not removing it when Task is not deleted.

[shivramsrivastava]

test case for RemoveTaskHelper is failed ,
Reason: While adding taskNode task is also added to the cost_model_ but when removing the Task it is not removed from the cost_model
//we are adding the task to cost_model when we addTask but not removing it when
//we call removeTask node BUG
cost_model_->RemoveTask(task_node->id_);