It can be found in ACP -> Tools -> Rebuild Caches
If you have a lot of user groups and permission combinations and have made changes over time, moving members in and out of groups, etc. then there may be some redundant data left in the database.
During testing on my site, there were 109 permission combinations with 71 unused and no longer needed.
After running the clean up, this was reduced to 38 permission combinations with 0 unused.
So a significant reduction in the total number of combinations.
Running this clean up task may yield significant speed improvements when creating new (private) nodes for example.
The queries were included in an earlier beta release upgrade script so anyone who has upgraded since B3 will already be benefitting from this improvement.