Revert "bindings: python: fix out-of-tree build"

Revert "bindings: python: fix out-of-tree build"

This reverts commit addf968c7321132a8c659e06cc06c76534ec31f5.

We're moving towards being compatible with PEP 517 for building and
currently the following error happens when running setup.py build_py:

running build_py
error: Error: setup script specifies an absolute path:

    <snip>/libgpiod/bindings/python/./gpiod/ext/chip.c

setup() arguments must *always* be /-separated paths relative to the
setup.py directory, *never* absolute paths.

As the Makefile build should only be used for development purposes, I
think we can safely drop support for out-of-tree build. Python bindings
have now been spun out into their own tarball and are available to
install from pip.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>