Remove unused $items member
Follows-up fad51e0cdbd (Ib479ae59) - which added addItem() as a way
to add items statically without needing to use the 'XAnalyticsSetHeader' hook
or needing to have the information in time before the hook runs.
However, the logic to add items after the hook is sent was never completed
and doesn't work.
Removing instead of fixing since it has been over 2 years and there haven't
been any issued filed about it.
Bug: T92875
Change-Id: I1f452546df363d15dfd6640e704e7258dec8e059