fix unused variable in the iterator

src/scripting/lua_terrainmap.cpp

@@ -406,7 +406,7 @@ static int impl_terrainmap_iter(lua_State* L)
 
 int intf_terrainmap_iter(lua_State* L)
 {
-	gamemap_base& tm = luaW_checkterrainmap(L, 1);
+	luaW_checkterrainmap(L, 1);
 	bool with_border = lua_isboolean(L, 2) ? luaW_toboolean(L, 2) : false;
 	lua_settop(L, 1);
 	luaW_pushlocation(L, map_location(with_border ? -1 : 0, 1, wml_loc{}));