Instead of handling the URCs in the modem object and using the
MMIfaceModem as a bridge to report the status read from the URC to a
call obtained from the MMCallList... just handle the URCs in the call
object itself.
Call information only lives in the ModemManager logic, there is no
associated date stored within the device itself. Therefore, simplify
everything by assuming there is nothing to remove.
Looks like this logic was implemented because it was originally based
on the SMS management logic, but for SMS we do have to remove
them (the stored PDU parts) from the device.
