resolv: remove resolv_set_private_dns_for_net

It is no longer necessary since ResolverController can directly
access the instance of PrivateDnsConfiguration and call its
method.

Bug: 130686826
Test: resolv_integration_test passed
Test: Switched to each private DNS mode and browsed websites
Change-Id: I944aa87c903c985d47466549c1b64034bdcaedd2