Various style cleanups + unused code removal

-> Don't pass pointer to whole IGameDef to NodeMetadata constructors
	and deserializers, but only to IItemDefManager, which is needed
-> Remove the unused content_mapnode_get_new_name() method
-> Fix style for MapBlock::deSerialize and MapBlock::deSerialize_pre22,
	improving accuracy of error messages a bit
-> Fix style at other serialisation methods too
-> Improve accuracy of some comments

src/content_mapnode.h

@@ -34,8 +34,4 @@ MapNode mapnode_translate_to_internal(MapNode n_from, u8 version);
 class NameIdMapping;
 void content_mapnode_get_name_id_mapping(NameIdMapping *nimap);
 
-// Convert "CONTENT_STONE"-style names to dynamic ids
-std::string content_mapnode_get_new_name(const std::string &oldname);
-class INodeDefManager;
-
 #endif