You cant. Its part of the Operating System, and windows uses the hardware acceleration in DX12 to draw the desktop and all the windows. If you uninstalled DX12 windows would not be able to draw.
Which is why It should not be trying to install it on a windows 10 box to begin with.
Can you get a copy of the Launcher from someone. Just copy the files? When you run it it will update itself to latest. Have you tried to install an earlier version? See if you can download and install 4.17?