[MLIR][Python] use FetchContent_Declare for nanobind and remove pybind (#161230)

Inspired by this comment
llvm/llvm-project#157930 (comment)
(and long-standing issues related to finding nanobind/pybind in the
right place), this PR moves to using `FetchContent_Declare` to get the
nanobind dependency. This is pretty standard (see e.g.,
[IREE](https://github.com/iree-org/iree/blob/cf60359b7443b0e92e15fb6ffc011525dc40e772/CMakeLists.txt#L842-L848)).
This PR also removes pybind which has been deprecated for almost a year
(llvm/llvm-project#117922) and which isn't
compatible (for whatever reason) with `FetchContent_Declare`.

---------

Co-authored-by: Jacques Pienaar <[email protected]>

Author: makslevental