Remove the default_do_language parameter, instead making DO use a hardwired

default of "plpgsql".  This is more reasonable than it was when the DO patch
was written, because we have since decided that plpgsql should be installed
by default.  Per discussion, having a parameter for this doesn't seem useful
enough to justify the risk of application breakage if the value is changed
unexpectedly.

src/test/regress/sql/plpgsql.sql

@@ -3164,12 +3164,12 @@ BEGIN
     LOOP
         RAISE NOTICE '%, %', r.roomno, r.comment;
     END LOOP;
-END$$ LANGUAGE plpgsql;
+END$$;
 
 -- these are to check syntax error reporting
 DO LANGUAGE plpgsql $$begin return 1; end$$;
 
-DO LANGUAGE plpgsql $$
+DO $$
 DECLARE r record;
 BEGIN
     FOR r IN SELECT rtrim(roomno) AS roomno, foo FROM Room ORDER BY roomno