This will remove a Custom XBert permanently, be sure that you don't want to Edit the Custom XBert instead.
From the Work board, go to Automation then XBert Automation.
Select the Custom XBerts button on the header. This will only show all Custom XBerts.
Select the Custom XBert you're looking to delete.
The Custom XBert Details Tile will open up.
Click on the Bin icon to delete.
You'll see this warning popup. Then you can hit Delete once you're sure.
Note - Once you hit Delete, it will automatically delete all the XBerts linked to the Custom XBert.