PL/Python: Update guide to alternative expected files

plpython_unicode_3.out was already removed a long time ago, so it
being listed here was very out of date.

plpython_types_3.out was removed with the Python 2 removal.

Author: petere

src/pl/plpython/expected/README

@@ -1,8 +1,3 @@
 Guide to alternative expected files:
 
 plpython_error_5.out			Python 3.5 and newer
-
-plpython_unicode.out			server encoding != SQL_ASCII
-plpython_unicode_3.out			server encoding == SQL_ASCII
-
-plpython_types_3.out			Python 3.x