wifi: remove unused hidden Hotspot 2.0 Release 1 API

WifiManager#buildWifiConfig was used by the Hotspot 2.0 Release 1
app (WifiInstaller) for parsing Release 1 installation data into
a WifiConfiguration object.  WifiInstaller will be updated to use
the new framework API (android.net.wifi.hotspot2.ConfigBuilder)
instead. So this API is no longer needed.

Bug: 32509661
Test: frameworks/base/wifi/tests/runtests.sh
Change-Id: Iacee739a9ed12ea00b1b7457e22e790ef4a177da