plpython: Code cleanup related to removal of Python 2 support.

plpython: Code cleanup related to removal of Python 2 support.

Since 19252e8ec93 we reject Python 2 during build configuration. Now that the
dust on the buildfarm has settled, remove Python 2 specific code, including
the "Python 2/3 porting layer".

The code to detect conflicts between plpython using Python 2 and 3 is not
removed, in case somebody creates an out-of-tree version adding back support
for Python 2.

Reviewed-By: Peter Eisentraut <peter@eisentraut.org>
Reviewed-By: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20211031184548.g4sxfe47n2kyi55r@alap3.anarazel.de