Camera: Fix PreviewSchedulerTest flake

When multiple preview buffers are queued together, it's still possible
choreographer callbacks occur in between, overriding one of the queued
buffers. Remove the corresponding test case.

Test: Run cameraservice_test for 20 iterations
Bug: 200306915
Change-Id: I3f46834ee9d2e67a138378e608f8929ddc0f7767