Removed leftover DNXCore50 reference

src/Discord.Net/Helpers/Shared/TaskHelper.cs

@@ -7,11 +7,7 @@ namespace Discord
 		public static Task CompletedTask { get; }
 		static TaskHelper()
 		{
-#if DNXCORE50
-			CompletedTask = Task.CompletedTask;
-#else
 			CompletedTask = Task.Delay(0);
-#endif
 		}
 	}
 }