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

[Maksim Levental via llvm-commits]

makslevental wrote:

Revert here https://github.com/llvm/llvm-project/pull/162309 to give people a chance to weigh in and catch up.

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