plpython: Remove obsolete test expected file

Move plpython_error_5.out to plpython_error.out, since the pre-3.5
version is no longer needed, since we raised the Python requirement to
3.6 (commit 45363fc).

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/d620e7c6-becc-4a8e-9b43-eea0da55faf2@eisentraut.org

Author: petere

src/pl/plpython/expected/README

@@ -243,7 +243,7 @@ $$
 plpy.nonexistent
 $$ LANGUAGE plpython3u;
 SELECT toplevel_attribute_error();
-ERROR:  AttributeError: 'module' object has no attribute 'nonexistent'
+ERROR:  AttributeError: module 'plpy' has no attribute 'nonexistent'
 CONTEXT:  Traceback (most recent call last):
   PL/Python function "toplevel_attribute_error", line 2, in <module>
     plpy.nonexistent