test: Remove driver_test.c

test: Remove driver_test.c

The driver_test.c driver wrapper (-Dtest in wpa_supplicant and
driver=test in hostapd) was previously used for testing without real
Wi-Fi hardware. mac80211_hwsim-based tests have practically replaced all
these needs and there has been no improvements or use for driver_test.c
in a long while. Because of this, there has not really been any effort
to maintain this older test tool and no justification to change this
either. Remove the obsoleted test mechanism to clean up the repository.

Signed-off-by: Jouni Malinen <j@w1.fi>