hearing_aid: Remove changes for the flag asha_higher_flush_threshold

The change is superseded by asha_asrc which sets an even higher
flush threshold. asha_asrc is now shipped and this change
is no longer required.

Bug: 308684052
Bug: 315500864
Test: m com.android.btservices
Change-Id: If7c92482efb1bd34be4056ee80b40d05af0ce792