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

[Konrad Kleine via llvm-commits]

kwk 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=$(python -m nanobind --cmake_dir)` and that branch will fire?

This should be the default and not opt-in.

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