Note: The following information applies to IS910.x

You cannot uninstall the CustomKey app. If you do not use the Custom Keys M1 or M2, you can delete the pre-installed key function:

Open the app list by swiping upwards from the bottom of the display.
Start the CustomKey1 or CustomKey2 app.
Select <Null> to delete the pre-installed key function.

Another option is to deactivate the CustomKey app. 

For this, you need a device management software, a so-called MDM tool.

Would you like to learn more about MDM tools? MDM on Wikipedia