plpython: Remove commented out code

This code has been commented out since the first commit of plpython.
It doesn't seem worth keeping.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: li carol <carol.li2025@outlook.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://www.postgresql.org/message-id/f31333f1-fbb7-4098-b209-bf2d71fbd4f3%40eisentraut.org

src/pl/plpython/plpy_plpymodule.c

@@ -153,8 +153,6 @@ PLy_init_plpy(void)
 
 	PyModule_Create(&PLy_module);
 
-	/* PyDict_SetItemString(plpy, "PlanType", (PyObject *) &PLy_PlanType); */
-
 	/*
 	 * initialize main module, and add plpy
 	 */