Bass: Remove sink broadcast metadata in source switch

Follow up on "Add support for multiple sources per sink device"

Sink metadata must be removed as it is no longer valid.
If not removed, the sink may attempt to synchronize with it
during resume (for another broadcast) or reconnection.

Bug: 385389212
Flag: EXEMPT, day-to-day bugfix covered by unit tests
Test: atest BassClientServiceTest
Change-Id: Icbf747c8fc4321a32ce830c79b340d8363b7ae24