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

[Maksim Levental via llvm-commits]

makslevental wrote:

> This should be the default and not opt-in.

If it's the default and not opt-in then the project cannot be successfully configured by default since the requirement has been removed from `requirements.txt`.

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