Remove VPN info arrays from NetworkStats(Observer|Recorder)

This change removes the now-unused VPN arrays in the network stats
observer and recorder classes. These are always null values in every
call site.

Bug: 113122541
Bug: 120145746
Bug: 129264869
Bug: 134244752
Test: FrameworksNetTest passing
Test: Manual tests show data usage fixes maintained.
Change-Id: Ieb8645acc400fdaeb0df7092c5369b96f9f35af9