How to remove unwanted performance overlay in CS:GO?

So, about a week ago I saw that in CS:GO, a random FPS + Min + AVG + Max + CPU + GPU always shows up on the upper-left side of my screen. When I press Shift + Tab, it changes to the upper-right side. Pushing it again moves it to the lower-right side. Again, lower-left side! It irritates me because it's not only in the main menu, but also when I am in a game...

I have noticed that when I press Shift + O it toggles it on or off, but I would rather not have it at all. How can I disable these shortcuts?

CS:GO does not have such a performance overlay option by default. You must have either added a net_graph configuration or installed another program like Afterburner/RivaTuner. I can't be sure without seeing a screenshot but it sounds more like a weird net_graph setup. If it is a program though, you should be able to easily disable that in its options or just uninstall it.

net_graph is a console command that has to be run or bound to a key every time CS:GO runs. The only way I know to do this is through an autoexec.cfg file.
This should be located:
C:\Program Files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive\csgo\cfg\autoexec.cfg
Although it will depend on where you installed CS:GO.

Once you locate that file, disabling it will be as simple as just deleting the file. I assume you copied that file there for a reason though, so if there's some other configuration in there you want, you should open it and just remove any lines with net_graph in them.