remove unused step counts

com.unity.ml-agents.extensions/Runtime/MultiAgent/BaseMultiAgentGroup.cs

     /// </summary>
     public class BaseMultiAgentGroup : IMultiAgentGroup, IDisposable
     {
-        int m_StepCount;
-        int m_GroupMaxStep;
         readonly int m_Id = MultiAgentGroupIdCounter.GetGroupId();
         HashSet<Agent> m_Agents = new HashSet<Agent>();