You have it sort of backwards. The BIOS default for a modern motherboard with integrated graphics is the integrated graphics with automatic recognition of a discrete graphics card.
So - you build the computer and plug the monitor into the motherboard. You get video.
Then you upgrade the graphics. There's no need to change anything here. The BIOS will automatically recognize the presence of the video card.
Remove the video card and the board defaults back to the onboard graphics.
Your problem is that you messed with the BIOS. Without the video card, you may not be able to get into the BIOS.
So, try it and see. Connect the monitor to the motherboard. Boot. If you can POST, you do not need to do anything. Your system should boot.
If you cannot POST, simply do what you need to reset the BIOS back to factory specs..