Bluetooth: Audio: Fix issues with unicast audio group lifetime

The unicast audio group can not have streams removed after the
CIG has been created as part of the QoS set procedure.

The bt_audio_unicast_group_remove_streams function may
leave the unicast group without streams, and thus the
check for "free" group was not correct, and has been replaced
with a simple boolean value.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>