Disallow object:remove() if the object is a player

Rebased by Zeno- (conflict in lua_api.txt)

doc/lua_api.txt

@@ -2294,6 +2294,7 @@ This is basically a reference to a C++ `ServerActiveObject`
 
 #### Methods
 * `remove()`: remove object (after returning from Lua)
+    * Note: Doesn't work on players, use minetest.kick_player instead
 * `getpos()`: returns `{x=num, y=num, z=num}`
 * `setpos(pos)`; `pos`=`{x=num, y=num, z=num}`
 * `moveto(pos, continuous=false)`: interpolated move

src/script/lua_api/l_object.cpp

@@ -131,6 +131,7 @@ int ObjectRef::l_remove(lua_State *L)
 	ObjectRef *ref = checkobject(L, 1);
 	ServerActiveObject *co = getobject(ref);
 	if(co == NULL) return 0;
+	if(co->getType() == ACTIVEOBJECT_TYPE_PLAYER) return 0;
 	verbosestream<<"ObjectRef::l_remove(): id="<<co->getId()<<std::endl;
 	co->m_removed = true;
 	return 0;