Reverting 2008 mac back to original OS from Prime OS

If you wiped out the local recovery mode partition, then you only have two options left.

1. Use the OS X 10.6 Snow Leopard Retail Upgrade DVD (one with the picture of the Snow Leopard on the DVD) so you can get a base OS installed. You can then use any other computer or OS which can access the Internet to download the macOS 10.11 El Capitan installer which you can then transfer to this iMac so you can install & upgrade it.
2. If you have access to another working Mac model generally from 2007 to 2015, then you can use it to create a bootable macOS USB 10.11 El Capitan USB installer. You can use the information in the following article to determine if the other Mac is also compatible with macOS El Capitan since those models I mentioned are just general guidelines: https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

You would have had a much easier time if you had planned ahead on how to get macOS reinstalled if you disliked your customization with a third party OS.

If nothing we've suggested here works for you, then you may be able to purchase a used copy of the Snow Leopard Upgrade DVD (one with the cat on it....not the gray label discs). Otherwise, you should consider installing Linux Mint on this iMac.

Hi,

If your internal drive still has Base System partition, Recovery mode is your friend.

How to start up from macOS Recovery – Apple Support (AU)

If it doesn't, and if you have another Mac, you could install macOS, Lion would be better on external drive with another Mac and boot from it on 2008 MacBook to install macOS on internal drive could works.