broadcast: Remove redundant broadcast start

Since Audio Framework driven BIG creation was introduced, there is no
need for external call for starting broadacst. Broadcast start is now
driven by Audio Framework callbacks.

Bug: 369554930
Bug: 347204335
Test: atest LeAudioBroadcastServiceTest
Test: atest bluetooth_test_broadcaster bluetooth_test_broadcaster_state_machine
Flag: leaudio_big_depends_on_audio_state
Change-Id: I02db58b85f1f117115845431ed7990cab4328554