Refactor send_to_where().

Refactor send_to_where().

Previously it had lengthy list of recognized query nodes to find out
unrecognized query quickly. The list must be updated if PostgreSQL
adds new query node. Since the list does very small contribution to
performance with large maintenance pain, I decided that keeping the
query node list is not worth the trouble and remove the list.

Also the second argument of send_to_where() is not actually used, I
remove the argument.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2023-May/004332.html