[TestOnly]Remove RemoteResponder from TetheringTester

This is no-op change. Originally RemoteResponder design for providing a
base responder that user could extend it like DnsServerResponder,
TcpResponder, UdpResponder. But actually user could just simply extend
TetheringTester that make RemoteResponder useless. Thus, remove it to
simply the code.

Test: atest EthernetTetheringTest
Change-Id: I47b22a1460ca312c678778143033aadf5e3e0b82
2 files changed