Hi @Thileepan,
Can you help me understand why you want to remove the default channels? You should be able to rename the default channels from the dropdown menu beside the channel name if that helps.
In the future we want to add the option to add additional default channels, or an option to choose which channels a user should be added to when they are invited to a team. If that sounds like something that's similar to what you want, we would welcome a feature request so others can upvote it too.