[le audio] Remove external source without local metadata

When a newly connected device has an existing source, the assistant doesn't have current metadata and it can't remove this source from remotes.
Assistant should also be able to remove the source without metadata.

Also this CL fixed one test error with bad code.

Flag: EXEMPT, trivial change covered by unit tests
Bug: 330690784
Test: atest BassClientServiceTest BassClientStateMachineTest
Test: manually test with newly connected device
Change-Id: I99bcffbf3868c21e52365862ceaf9de98538f956