Remove no-longer-needed old-style check for incompatible plpythons.

Commit 866566a6 introduced a new mechanism for incompatible
plpythons to detect each other.  I left the old mechanism in place,
because it seems possible that a plpython predating that commit might be
used with one postdating it.  (This would require updating plpython3 but
not plpython2 or vice versa, but that seems well within the realm of
possibility.)  However, surely it will not be able to happen in 9.6 or
later, so we can delete the old mechanism in HEAD.