That partial is on all pages that display a list of topics ie /recent /unread /popular and /category pages. You need to create a plugin and add hooks for those pages the hook format is filter:<template_name>.build in those hooks you need to go through the topics array and subtract one from each topic's postcount.
[[email protected] community]$ node app.js
info: NodeBB v0.4.2 Copyright (C) 2013-2014 DesignCreatePlay Inc.
info: This program comes with ABSOLUTELY NO WARRANTY.
info: This is free software, and you are welcome to redistribute it under certain conditions.
info: Time: Mon May 12 2014 06:31:33 GMT+0400 (MSD)
info: Initializing NodeBB v0.4.2
info: * using configuration stored in: /var/www/alternative-gaming/community/config.json
info: * using mongo store at 127.0.0.1:28900
info: * using themes stored in: /var/www/alternative-gaming/community/node_modules
Failed to load c++ bson extension, using pure JS version
error: auth failed