As Scott & pupi1985 said, Q2A and most of the theme and plugins released under GPL.
You are free (legaly) to edit any part of code of the products. If you are keeping the credit links on your website, it really helps the community / product to grow by spreading it's name which is really appreciated.
If you want to remove it for some reason, then feel free to do so. No one is going to chase you for doing this.
If you want to make a donation to support the development, you are welcome. It would be very encouraging. If you would like to make a donation for Donut theme, please PM me.