Floss: Remove CallSource from CallInfo

Since "Floss: add workaround entry to insert call when sco start"
(commit 9d9ab011) the CallSource in call list is no longer needed.
This commit removes it to make the code cleaner.

It also prevents the creation of the UHID device and SCO link
disconnection on AT+CHUP reception for the devices requesting a +CIEV
event to start the SCO link.

Bug: 328696624
Test: Conduct the following manual tests.
  pair supported Bluetooth Headset
  load https://google.github.io/libhidtelephony/ with Chrome
  click on "Start", select the headset then "Connect"
  "Inputs Reports" and "Output Reports" are correct
  "Hook Switch", "Off-Hook", "Ring" and "Mute" test cases are passed
  used https://online-voice-recorder.com during "Hook Switch" and
    "off-Hook" tests to check audio input
  Google Meet: headset is able to mute, un-mute and hang-up the call
Test: Conduct the following manual tests.
  pair unsupported Bluetooth Headset or add it to
    INTEROP_INSERT_CALL_WHEN_SCO_START in
    /var/lib/bluetooth/interop_database.conf.
  load https://google.github.io/libhidtelephony/ with Chrome
  click on "Start", the headset is not proposed
  Google Meet: headset is usable but not able to mute, un-mute or hang-up
    the call
Test: atest bluetooth_test_gd
Tag: #floss
Flag: EXEMPT, no behavior change on Android; Floss-only changes

Change-Id: If5ac3206cdca4607f62e636014729c1b9b6198a7