Ignore/deprecate "number" field for broadband modems

The "number" field in bearer properties is currently a legacy configuration that we receive from NetworkManager (set to *99# usually). It was used in MM 0.6 and earlier when connecting AT+PPP based modems, and since MM 1.x this field is completely ignored in broadband modems. This field has been left around because if ModemManager ever supports POTS modems we'll need it so that the user can use it to report which number to call to.

Given that broadband modems don't require it, we should ignore it completely and avoid showing it in the bearer properties.