[llvm] [mlir] [MLIR][Python] use `FetchContent_Declare` for nanobind and remove pybind (PR #161230)

[Maksim Levental via llvm-commits]

makslevental wrote:

> Simply get rid of the `REQUIRED` option to `find_package`.

I don't understand how this helps you if you don't want to use fetch content - you can already set `-Dnanobind_DIR` and that branch will fire?

https://github.com/llvm/llvm-project/pull/161230