bpo-45570: Simplify setup macros for pyexpat (GH-29159)

* ``HAVE_EXPAT_CONFIG_H`` is not used by our code and not used by
  system-wide expat header files
* ``USE_PYEXPAT_CAPI`` is no longer used by our code
* ``XML_POOR_ENTROPY`` should be defined in expat_config.h

Signed-off-by: Christian Heimes <christian@python.org>