layers: Don't use instance pointer after it has been destroyed

layers: Don't use instance pointer after it has been destroyed

In unique_objects' explicit_DestroyInstance(), the recently introduced
second get_dispatch_table() was using a destroyed instance as a lookup
key, failing to find an entry (since the driver controls that memory
and probably freed it), and was crashing consistently on PixelC.

Instead, use the instance pointer to cleanup instanceExtMap before
calling down to the ICD's DestroyInstance.

Additionally, use the existing VkLayerInstanceDispatchTable* instead
of calling get_dispatch_table() twice.