When "average generation time" is zero, hide the sparkline and "Avg generation time" graphs metric from picker #17010
Description
As a user, when i'm viewing a period where the "average generation time" is zero, for example under "Visitors > Overview", it would be helpful if:
- the sparkline "0 average generation time" is hidden
- and "Avg generation time" metric is removed from graphs metric picker
Context: currently this metric is shown for Matomo instances that used to track this metric (before Matomo 4). Which is helpful, but in this issue we just want to remove any confusion and hide the feature when it's definitely zero for the selected period.