Resynchronise statistics does not resynchronise number of users in groups

After rampant growth of spambot registrations and laziness on my part I recently had to remove some 3000 spambots. This wasn't possible to prune easily using the ACP prune user function as it would freeze the browser. Trying to limit the prune to 100 users didn't work either as the server would timeout after processing ~40 user deletions, which made the process tedious (this is probably a bug in itself). In the end I just removed all users with 0 posts with a greater user ID than the bots directly using SQL.

The next step I did was to resynchronise the statistics to update the user count. However, this did not affect the user count of groups and the groups still contain the non-existing users. This breaks the pagination the pagination code expects it to be some 3200 users while there is only 200 users, which inserts a lot of blank pages.