Remove another ss_active_users check from SiteStats::isSane

Also add a comment to method doc to make sure, nobody will add it again.

Follow-Up: Id8f1cb7035ae0058fb34a6c7bdf7bf9f6c740a69
Follow-Up: I268599be96106e1175fdf9750a2adc9468ebc93c
Change-Id: Ifd6da554b531720aace4497f7fbc2f58c12edc07
Merge "Remove another ss_active_users check from SiteStats::isSane"