Possible fix for unable to boot without GPU/graphic card/constant reboot failure [READ DESCRIPTION]
If you're unable to boot after your GPU/graphics card is removed from the system, go in the BIOS and try to disable C3/C6 state support in the menu "Frequency settings/Advance" and then "Advanced CPU core features/CPU Power Configuration"
[this varies widely between Motherboard brands, try to look for the C3/C6 state options if you can't find these labels].
If it still doesn't work, try disabling C1E as well (I had to for my case).
I do not have the technical details on why this worked or what the underlying root cause it but it may or may NOT indicate an issue with your Power Supply Unit.
EDIT 24July2019: Just realized like video kinda gotten alot of clicks. Added a more detailed description
My original thread: https://linustechtips.com/main/topic/593880-old-rig-not-booting-without-gpu/
Full credits: http://www.tomshardware.com/forum/314208-30-wont-boot-windows-intel-graphics#10737012