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

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

We still have a lot of usage of pybind, some of which is fairly simple to migrate, but much of which relies on some bindings/type coverters we haven't implemented for nanobind yet (e.g. `absl::StatusOr`, protos). Also, I don't actually see the deprecation status anywhere -- was it announced or documented anywhere that pybind support was being removed? I'd like to request a revert, at least temporarily, so we have some more lead time to migrate things over.

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