Delete legacy "wifi interface name guessing" behaviour

Now that Wi-Fi always passes us the AP interface name (and mode)
we no longer need to guess which interface on which we're supposed
to be starting IP serving (either tethering or local-only hotspot).

Test: as follows
    - built
    - flashed
    - booted
    - TetheringTest passes
Bug: 32163131
Bug: 62343300

Change-Id: I6019410ee5adff4929690d35ba09294765fcd6a4