[le audio] Fix removing broadcast source when PA is still synced

When PA is still synced, assistant will trigger pending source remove operation with any RS update.
Pending remove should only be sent when both PA and BISes are unsynced.

Bug: 378532642
Flag: EXEMPT, trivial change covered by unit tests
Test: atest BassClientStateMachineTest
Change-Id: Icdf32676efdd6dec4f4049e66872b674b2883f23