Floss: Remove unnecessary check in HFP Hangup callback

The should_insert_call_when_sco_start check was introduced to prevent
accidentally remove calls created by CRAS during SCO start. This was
necessary before aosp/3058085, when HFP hangup callback could change the
call state in the headset.

Since aosp/3058085, HFP callback no longer modify the call state.
Therefore, the should_insert_call_when_sco_start check is now redundant
and can be removed.

Bug: 331875859
Test: Conduct the following manual tests
  pair Bluetooth Headset
  Google Meet: headset is able hang-up the call
Test: Conduct the following manual tests
  pair Bluetooth Headset
  login Ring Central and place an incoming-call
  answer the call by headset
  hangup the call by headset
Test: Conduct the following manual tests
  pair Bluetooth Headset
  login Ring Central and place an incoming-call
  answer the call on Ring Central
  hangup the call from headset
Test: atest bluetooth_test_gd
Tag: #floss
Flag: EXEMPT, no behavior change on Android; Floss-only changes
Change-Id: I077c6f275274d2c0a378bf5f9812ffeeb31bc17e