When using NetworkInterface::get_default_instance() application gets handle
to CellularInterface which is actually CellularContext derived from CellularInterface.
Application needs also handle to CellularDevice to open other interfaces.
It was decided to retire CellularBase class which served as a pure
virtual interface class from which Cellular network stack
implementations would get inherited. However, the current view is that
we may be the only user of it so we could retire CellularBase.