tests: Make sigma_dut_dpp_qr_mutual_resp_enrollee_connector_privacy more robust

tests: Make sigma_dut_dpp_qr_mutual_resp_enrollee_connector_privacy more robust

Clear the dpp_connector_privacy_default parameter value that sigma_dut
set in wpa_supplicant at the end of the test case to avoid surprising
behavior for the following test cases. This was found with a failure in
the following test sequence:
sigma_dut_dpp_qr_mutual_resp_enrollee_connector_privacy
sigma_dut_dpp_proto_peer_disc_req

Signed-off-by: Jouni Malinen <quic_jouni@quicinc.com>