audio_hal_interface: Deprecate manually disabling the audio HAL

Remove the system property:
   persist.bluetooth.bluetooth_audio_hal.disabled

The property used to exists as *.bluetooth_audio_hal.enabled
and was switched when the audio HAL was enabled by default.
This property is no longer used in any meaningful way.

Bug: 365022887
Test: m com.android.btservices
Flag: EXEMPT, minor refactor
Change-Id: I071dce1cacd15797add7a77efc5f9a1d808334bb