WifiManager: remove last remnants of setWifiApEnabled

Now that the WifiManager.setWifiApEnabled call is deprecated and
removed, delete the underlying code.

Bug: 66917593
Test: frameworks/opt/net/wifi/tests/wifitests/runtests.sh
Test: frameworks/base/wifi/tests/runtests.sh
Change-Id: If68ddd3e2301b0ba9efc3914e2a0f0e17436ccf0