This post is to help me and other people that might have had this issue before (and mostly future me).

When you have installed an Nvidia GPU in Arch Linux with i3 as Window Manager (WM), you probably needed to install and nvidia or another similar package, which contains drivers and generally includes a tool that automatically creates/modifies the Xorg configuration file (Xorg.conf), for the correct operation of the GPU in Arch and i3.

Now the problem is that if you for some reason need to remove the GPU, then the Xorg.conf will still expect to use the GPU as device for rendering the Display.

First a bit of background,

When you start the X Windows System, an X session will be created. The standard ways of starting an X session are to run startx from console login or to login using a Display Manager (DM), such as lightdm (the DM I was using).

The solution is simple: Just remove any reference to nvidia on the Section corresponding to the Device.

One one you can test the Xorg.conf is in a sane state is by using

Your /etc/X11/Xorg.conf will look something like this:

Section "Monitor"
    Identifier      "Monitor0"
    VendorName      "Unknown"
    ModelName       "Unknown"
    HorizSync       28.0 - 33.0
    VertRefresh     43.0 - 72.0
    Option          "DPMS"
EndSection

Section "Device"
    Identifier      "Device0"
    Driver          "nvidia"
    VendorName      "NVIDIA Corporation"
EndSection

Section "Screen"
    Identifer       "Screen0"
    Device          "Device0"
    Monitor         "Monitor0"
    DefaultDepth    24
    SubSection      "Display"
        Depth       24
    EndSubSection
EndSection

You should modify it so that it looks like this:

Section "Monitor"
    Identifier      "Monitor0"
    VendorName      "Unknown"
    ModelName       "Unknown"
    HorizSync       28.0 - 33.0
    VertRefresh     43.0 - 72.0
    Option          "DPMS"
EndSection

Section "Screen"
    Identifer       "Screen0"
    Device          "Device0"
    Monitor         "Monitor0"
    DefaultDepth    24
    SubSection      "Display"
        Depth       24
    EndSubSection
EndSection