wifi: cancel autoscan timeout when removing device

Usually autoscan is cancelled when the device is disabled, but when
wpa_supplicant leaves D-Bus while device has an ongoing power request,
__connman_device_disable() will return early and thus autoscan timeout
isn't cleared. The timeout callback may then access released memory
when it's activated.

Add stop_autoscan() call to wifi_remove() to make sure there are no
pending timeouts when the autoscan struct is released.

[connman] wifi: cancel autoscan timeout when removing device