Various changes to vm_object_page_remove():

Various changes to vm_object_page_remove():
 - Eliminate an odd, special-case feature:
   if start == end == 0 then all pages are removed.  Only one caller
   used this feature and that caller can trivially pass the object's
   size.
 - Assert that the vm_object is locked on entry; don't bother testing
   for a NULL vm_object.
 - Style: Fix lines that are longer than 80 characters.