Open CS:GO and go to Settings> Video settings.
Set the aspect ratio as 4:3.
Set the the display mode as Fullscreen/Windowed.
Remember you must be in the old (Scaleform) UI
When you finished all the steps above now you should have a streched viewmodel without black bars!
Since CS:GO automatically sets the video settings by default for you, I recommend to change some for a better visual experience, since playing CS:GO stretched makes the game look worse.
Set Multisampling Anti-Aliasing Mode to the highest one you can (For me it's 8x MSAA)
Disable FXAA Anti-Aliasing
Set Texture Filtering Mode to the highest one you can (For me it's ANISOTROPIC 16x)