kernel - SWAP CACHE part 2/many - Remove VM pager lists

kernel - SWAP CACHE part 2/many - Remove VM pager lists

* VM pager lists were used to associate handles with VM objects.  Only the
  device_pager actually used them.  Store the VM object in cdev_t->si_object
  instead and remove the device pager's VM pager list.

* phys_pager and swap_pager only use anonymous objects, the VM pager lists
  were implemented but not used.  Assert that the handles are NULL and remove
  the VM pager lists.

* Remove vm_pager_object_lookup().