Update connectivity value on device removal

When a device is removed (like when the user unplugs a usb network device) the device object is removed, so it doesn't emit a notify signal for a change in its connectivity and so, device_connectivity_changed is not called. This means that nobody updates the global connectivity value which is potentially wrong if the device was the one providing network connectivity.

Since device_connectivity_changed's first two parameters aren't actually used and are there just for the signal to be able to be connected, I moved the code from device_connectivity_changed to a new update_connectivity_value function that just takes a NMManager parameter and also call it from remove_device.

Fixes #141 (closed)

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