Remove stale link to international site

Remove stale link to international site

The Polish site hasn't been updated in 10+ years with 9.3 being
the latest new item posted.  As there is no contact information
available to contact the site owner, remove for now to keep the
links to the set up updated sites.

Reported-by: Stefan Kaltenbrunner <stefan@kaltenbrunner.cc>
Reviewed-by: Jonathan S. Katz <jkatz@postgresql.org>
Reviewed-by: Magnus Hagander <magnus@hagander.net>
Discussion: https://postgr.es/m/ccbb7d4b-29c1-4622-84a8-72d1ae6a9786@kaltenbrunner.cc