Remove ExternalPrivateDnsStatus

ExternalPrivateDnsStatus was introduced for C-compatible version of
getStatus(), and now it's no more needed.

Bug: 130686826
Test: atest --include-subdirs packages/modules/DnsResolver
Test: dumpsys dnsresolver showed correct output
Change-Id: Iac50908f269d8921f1aadba8f2b50286a33325f7