Further work on pushing Giant out of the vm_map layer and down

Further work on pushing Giant out of the vm_map layer and down
into the vm_object layer:
 o Acquire and release Giant in vm_object_shadow() and
   vm_object_page_remove().
 o Remove the GIANT_REQUIRED assertion preceding vm_map_delete()'s call
   to vm_object_page_remove().
 o Remove the acquisition and release of Giant around vm_map_lookup()'s
   call to vm_object_shadow().