Avoid direct cross-module links in hstore_plperl and ltree_plpython, too.

Just turning the crank on the project started in commit d51924be8.
These cases turn out to be exact subsets of the boilerplate needed
for hstore_plpython.

Discussion: <2652.1475512158@sss.pgh.pa.us>

contrib/ltree_plpython/ltree_plpython2u--1.0.sql

@@ -3,11 +3,6 @@
 -- complain if script is sourced in psql, rather than via CREATE EXTENSION
 \echo Use "CREATE EXTENSION ltree_plpython2u" to load this file. \quit
 
--- make sure the prerequisite libraries are loaded
-LOAD 'plpython2';
-SELECT NULL::ltree;
-
-
 CREATE FUNCTION ltree_to_plpython2(val internal) RETURNS internal
 LANGUAGE C STRICT IMMUTABLE
 AS 'MODULE_PATHNAME', 'ltree_to_plpython';