do not spam warnings about unknown attribute...

[unit_type]do_not_list= which is never used by the engine (bug #17077)

src/unit.cpp

@@ -514,6 +514,7 @@ unit::unit(const config &cfg, bool use_traits, game_state* state) :
 	}
 
 	foreach (const config::attribute &attr, input_cfg.attribute_range()) {
+		if (attr.first == "do_not_list") continue;
 		WRN_UT << "Unknown attribute '" << attr.first << "' discarded.\n";
 	}
 }