btif_a2dp_source: Cleanup following removal of a2dp_source_threading_fix

Inline functions:
 - btif_a2dp_source_end_session_delayed
 - btif_a2dp_source_shutdown_delayed
 - btif_a2dp_source_cleanup_codec

Bug: 331817295
Test: m com.android.bt
Flag: EXEMPT, no logical change
Change-Id: I07c77cf2e58c27c4b5ebbcf7688fc2c785a223cd