Remove macros for older unity versions which are no longer needed. (#3168)

UnitySDK/Assets/ML-Agents/Editor/DemonstrationImporter.cs

                 var texture = (Texture2D)
                     AssetDatabase.LoadAssetAtPath(k_IconPath, typeof(Texture2D));
 
-#if UNITY_2017_3_OR_NEWER
-#else
-                ctx.SetMainAsset(ctx.assetPath, demonstration);
-#endif
             }
             catch
             {

UnitySDK/Assets/ML-Agents/Editor/Tests/StandaloneBuildTest.cs

 using System;
 using UnityEditor;
 using UnityEngine;
-#if UNITY_2018_1_OR_NEWER
-#endif
 
 namespace MLAgents
 {
         {
             string[] scenes = { "Assets/ML-Agents/Examples/3DBall/Scenes/3DBall.unity" };
             var buildResult = BuildPipeline.BuildPlayer(scenes, "testPlayer", BuildTarget.StandaloneOSX, BuildOptions.None);
-#if UNITY_2018_1_OR_NEWER
             var isOk = buildResult.summary.result == BuildResult.Succeeded;
             var error = "";
             foreach (var stepInfo in buildResult.steps)
                     }
                 }
             }
-#else
-            var error = buildResult;
-            var isOk = string.IsNullOrEmpty(error);
-#endif
             if (isOk)
             {
                 EditorApplication.Exit(0);

UnitySDK/Assets/ML-Agents/Scripts/Grpc/RpcCommunicator.cs

             var result = m_Client.Exchange(WrapMessage(unityOutput, 200));
             unityInput = m_Client.Exchange(WrapMessage(null, 200)).UnityInput;
 #if UNITY_EDITOR
-#if UNITY_2017_2_OR_NEWER
-#else
-            EditorApplication.playmodeStateChanged += HandleOnPlayModeChanged;
-#endif
 #endif
             return result.UnityInput;
 #else
         #endregion
 
 #if UNITY_EDITOR
-#if UNITY_2017_2_OR_NEWER
         /// <summary>
         /// When the editor exits, the communicator must be closed
         /// </summary>
             }
         }
 
-#else
-        /// <summary>
-        /// When the editor exits, the communicator must be closed
-        /// </summary>
-        private void HandleOnPlayModeChanged()
-        {
-            // This method is run whenever the playmode state is changed.
-            if (!EditorApplication.isPlayingOrWillChangePlaymode)
-            {
-                Close();
-            }
-        }
-
-#endif
 #endif
     }
 }