Remove unneeded notif vis checks

The visibility of ExpandableNotificationRows does not need to be checked
inside of NotificationStackSizeCalculator; it is already handled as part
of the Notification Pipeline (inside of KeyguardCoordinator). The View
layer (NSSL and associated classes, such as this one) can assume that
any Notifications that are present should actually be there.

Fixes: 226132311
Test: Verify multiple notifications visible on the lockscreen
Change-Id: I1117f3c139422a9110a9f5353fc27bbcbb534f75