Revert "Take all VPN underlying networks into account when migrating traffic for"

This reverts commit 95aa6d44.

Reason for revert: This change has been implicated in 4-way deadlocks as seen in b/134244752.

Bug: 134244752
Change-Id: I2f1839d7776a613ca571af8a542755ddc5fc8760
Merged-In: Ibdaad3a4cbf0d8ef1ed53cfab1e454b9b878bae9