VTS Audio Effect: Do not check the result of IEffect::offload method

HAL may not support offloading, and a call to IEffect::offload
is allowed to return an error code.

Update the test to only check that the call to the method doesn't
crash.

Bug: 38437110
Test: still passes
Change-Id: I6cfd25998edb9c68d2e585e572ba8ba3a3427e98