[llvm] [Offload] Don't add the unsupported host plugin to the list (PR #159642)

[Ross Brunton via llvm-commits]

https://github.com/RossBrunton approved this pull request.

Lgtm. This should also help with the issues I brought up with at https://github.com/llvm/llvm-project/pull/158900#issuecomment-3311770237 .

https://github.com/llvm/llvm-project/pull/159642