[postgres] remove node_pos index which is not immutable

This caused subtle bugs when ALTER TABLE with search_title was
called. It is a wrong practice to define an index immutable when in
fact it's not. The count(id) part makes it mutable in fact.

devops/postgres/schema.sql

@@ -210,5 +210,5 @@ CREATE OR REPLACE function node_pos(int, int) returns bigint
    LANGUAGE SQL immutable;
 
 --drop index node_by_pos;
-create index node_by_pos on nodes using btree(node_pos(id,typename));
+--create index node_by_pos on nodes using btree(node_pos(id,typename));