Fix flaky test VibrationThreadTest#vibrate_multipleVibrations_withCancel

We're removing the callback verification since the test's focus is not on verifying the onComplete callback's timing after a vibration cancellation. Instead, we aim to confirm that the final vibration, triggered post-cancellation, actually executes as expected.

Bug: 349156580
Test: atest --rerun-until-failure 1000 com.android.server.vibrator.VibrationThreadTest#vibrate_multipleVibrations_withCancel
Flag: TEST_ONLY
Change-Id: I80dd468c9f31a1676e38b24766388934a6f03fa7