Remove batch_size parameter from iter_messages
Remove batch_size parameter from iter_messages

It was only useful for testing purposes, and no other methods
exposed this kind of parameter (but they still use it).

LonamiWebs/Telethon,LonamiWebs/Telethon,LonamiWebs/Telethon,expectocode/Telethon,LonamiWebs/Telethon