[SP07] Remove reference of NetworkStatsService in IpServer

Currently NetworkStatsService is notified when downstream is
updated. However, it seems unnecessary given that tether stats
is persist since boot, and there is no any upstream change when
downstream is changed.

Test: atest NetworkStatsServiceTest IpServerTest
Bug: 130855321
Change-Id: Ie300bfeb0a04678fcfcf300843b6f859af9df91d