leaudio: Fix crash when removing device while streaming

With this patch:

1) Device is removed from the group when it is disconnected
2) Empty group is removed only when related CIG does not exist

Bug: 204945451
Bug: 150670922
Tag: #feature
Sponsor: jpawlowski@
Test: atest --host bluetooth_le_audio_test bluetooth_le_audio_client_test

Change-Id: I7079594253a504cd0320c6d4000c3d71cb6f0883