Remove duplication definitions of NAMEDATALEN.

Remove duplication definitions of NAMEDATALEN.

The definition for NAMEDATALEN is now in
src/include/pg_config_manual.h only.  Also replace POOL_NAMEDATALEN
with NAMEDATALEN in src/utils/pool_select_walker.c.

I tried to eliminate copying pool_config_manual.h from PostgreSQL but
it seems it is a little bit difficult to do that at this moment.
There are some definitions like pg_attribute_format_arg were added to
it.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2023-August/004366.html