layers: MR129, Remove incorrect asserts from param_checker and object_tracker

layers: MR129, Remove incorrect asserts from param_checker and object_tracker

For apps that run with multiple instances, it's perfectly valid to have instances
remaining at DestroyInstace() time so removing that assert from param_checker.
Also, the assert appears to have been cut-and-pasted into DestroyDevice of both
param_checker and object_tracker. The same logic applies for apps that use
multiple devices. Running vulkancts with layers enabled found these issues.