[xnnpack][executorch] remove unordered_set from xnn_compiler (#89231)
Removing unrodered_set from xnncompiler for executorch.
While some STL libraries are unavoidable, and I think it should be ok for delegate to pull these libraries, unordered_set wasn't really needed, and we should be serializing the number of external ids anyways
After this, the backend classes should be good to hg copy into executorch
Differential Revision: [D41227391](https://our.internmc.facebook.com/intern/diff/D41227391/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89231
Approved by: https://github.com/salilsdesai, https://github.com/cccclai
