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

[Maksim Levental via llvm-commits]

makslevental wrote:

> I also expected to see some sort of deprecation comment/annotation in mlir/include/mlir/Bindings/Python/PybindAdaptors.h itself.

That's a good idea - I'll add those since we reverted/restored that file.

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