First of all, I don't think the foreach-object should be necessary in this instance.
However, the error suggests that the computer(s) in question are not the final object in the tree. In other words, they may have other objects inside them.
If this is the case you could try Remove-ADObject with the -recursive parameter
More about this cmdlet here:
Does that not solve what you need to solve? I don't see the need to create a second CSV, and import the second CSV, and then do your deletes. Just do it all on the same pipeline. Unless I'm missing some critical step that happens in between the two, but I don't think I am.