Remove duplicate reading of offload tethering stats

The obtained interface stats already include offload tethering stats (https://cs.android.com/android/platform/superproject/+/master:packages/modules/Connectivity/service-t/src/com/android/server/net/NetworkStatsService.java;l=1903;drc=08ccd0d2e1b2eda92ea61d3061f86b881a4a0ef6)

Additionally, the removed code no longer compiles since it was moved to the Connectivity module.

Change-Id: I380e529011cdf108f009d8e5fb35f849fc7e3963