I think I no longer want to be the godfather to my friend's child.
Why don't you want to be their godfather any longer?
I think it's too much responsibility.
That role, even if mostly just a title, has importance and if you are not comfortable with it then you need to be upfront and honest about your feelings. Being a godfather is not for you and understanding that is important.
What do you think I should say?
You should just tell them that you don't think you're the right person to be the godfather, and that they should choose someone else. I'm sure your friend will understand.