Remove sink_audio_policy_handover flag

The change is in nextfood and rolled out to public, so remove the flag

Bug: 305636605
Bug: 333562819
Change-Id: I27ee4a3eb728671b92b16505e78afe3706fe5a80
Test: atest HeadsetServiceTest
Flag: com::android::bluetooth::flags::sink_audio_policy_handover