I want to remove the default channels which are created while creating a team

Hi @lioman, that makes sense - would you be able to open a feature request on it?

As a workaround for Off-Topic channel, I think if you:
1. Rename the original Mattermost Off-Topic channel to something else and change the channel URL
2. Rename "random" to Off-Topic and change the channel URL to off-topic

Then your "random" channel history will now show up as the default Off-Topic channel. We have a ticket here to also let the URL of town-square be changed, but it's currently not possible.