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
Bug: 133117690
Change-Id: I16b06d9f8d438950116696296a6d6a066eb4c08b
Merged-In: I340f60ef11415c9b83aa66884fffdfae3153d1d0
(cherry picked from commit 83c038a3)