Move pybind11 from submodule to pyproject.toml:build-system.requires.

[hzhangxyz]

Since we can specify setup dependencies in pyproject.toml, pybind11 could be added here instead of placed directly under the root of the repo, which make the repo structure more compact. And the path of pybind11 cmake config could be set by pybind11_DIR in setup.py.