bass_client: Don't remove source on stream suspension

This change allows sink devices to take responsibility over maintaining
source synchronization while requesting unicast streaming.

Tag: #bug
Bug: 330690507
Bug: 333050419
Test: atest BassClientServiceTest
Test: atest BassClientStateMachineTest
Change-Id: I0d5a01c72aac22a88e3623b6bd2e96d42eabdfd2