Revert "TetherController - populate TetherUpstream6Map"

This code is moving to BpfCoordinator in this topic.

This reverts commit 419b1d4a.

Test: manual testing of IPv6 upstream tethering
Test: dumpsys tethercontroller shows upstream map non-empty
Change-Id: Ia78bc4df11080df0605484dd7472f2c69a3a12d4