Move clientsimpleobject.h to client folder (#9630)

This file is only called from client folder, retrieve its friends :)

src/client/clientsimpleobject.h

@@ -0,0 +1,35 @@
+/*
+Minetest
+Copyright (C) 2013 celeron55, Perttu Ahola <celeron55@gmail.com>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#pragma once
+
+#include "irrlichttypes_bloated.h"
+class ClientEnvironment;
+
+class ClientSimpleObject
+{
+protected:
+public:
+	bool m_to_be_removed = false;
+
+	ClientSimpleObject() = default;
+	virtual ~ClientSimpleObject() = default;
+
+	virtual void step(float dtime) {}
+};