ofono: Remove network if GPRS context is removed

ofono: Remove network if GPRS context is removed

Whenever the GPRS contextext is removed we need also to remove
the network object. Following sequence was observed:

  cm_update_attached() /huawei_1 Attached 1
  modem_changed() /huawei_1 Interfaces 0x07

Plugin creates network object and registers it at core

   cm_update_attached() /huawei_1 Attached 0
   service.c:run_auto_connect()
   set_property() /huawei_1 path /huawei_1/context1 org.ofono.ConnectionContext.Active

Obviously, the context is not attached at this point so we get an

   Failed to change property: /huawei_1/context1 org.ofono.ConnectionContext.Active: org.ofono.Error.NotAttached GPRS is not
attached

Fixes BMC#25725