This CL removes the fuchsia.mdns FIDL API definition and transitions
the implementation and all clients to the new fuchsia.net.mdns API.

This change breaks the ability to generate traffic logs, the
SetVerbose method being absent from the new FIDL definition. This
capability will be restored, probably via /hub.

TEST: no behavior change, multiple regression tests in CQ

Change-Id: I6438f0a8fd316efa33a87908d41b8f9961e523d6