[usecase-webapi] Improve webaudio usecase

- Remove precondition.

Impacted TCs num(approved): New 0, Update 1, Delete 0
Unit test Platform: [Andriod]
Test result summary: Pass 1, Fail 0, Blocked 0
Merge pull request #611 from JianfengXu/xujf/webaudio/update

[usecase-webapi] Improve webaudio usecase