plpython: Remove support for major version conflict detection

This essentially reverts commit 866566a690b, which installed safeguards
against loading plpython2 and plpython3 into the same process. We don't
support plpython2 anymore, so this is obsolete.

The Python and PL/Python initialization now happens again in _PG_init()
rather than the first time a PL/Python call handler is invoked. (Often,
these will be very close together.)

I kept the separate PLy_initialize() function introduced by 866566a690b
to keep _PG_init() a bit modular.

(I'm looking at the some further work to allow multiple interpreters in
PL/Python, so I'm trying to clean up the initialization code a bit first.)