You have to go to the device list, and select the device you can remove.
On the menu "More", select "Remove and uninstall" (picture 1).
The device will change the status to "Uninstalling" (picture 2).
If you online the device, the device will uninstall Avast and it will disapear from the device list.
If the device is not going to be online for the uninstall task, you have to go to the menu "more" and select "Remove" (picture 3).