As the others have said:
You don't need to uninstall anything. There is no uninstaller to remove Mint.
If you're planning on reinstalling - literally just use your Mint live USB and reinstall over the top of the existing installation.
But if you REALLY insist that you must completely remove it - for whatever reason - you could perhaps boot into your live USB and then use something like dd to overwrite the entire drive, or partition where your mint installation resides with zeros from /dev/null.
Then you could use something like gparted, or some other disk management tool to reformat the entire partition.
(I don't use mint, so I don't know exactly what's on the Live environment, by default)
Or you could download DBAN (Dariks Boot and Nuke), or some other data removal tools and burn them to a disk, or to a USB drive and use those to wipe your HD!