resolv: remove resolv_delete_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: Re-enabled WiFi, and browsed websites
Change-Id: I340f60ef11415c9b83aa66884fffdfae3153d1d0