Optimize vm_page_try_to_free().  Specifically, the call to pmap_remove_all()
can be avoided when the page's containing object has a reference count of
zero.  (If the object has a reference count of zero, then none of its pages
can possibly be mapped.)

Address nearby style issues in vm_page_try_to_free(), and change its
return type to "bool".

Optimize vm_object_page_remove() by eliminating pointless calls to
pmap_remove_all().  If the object to which a page belongs has no
references, then that page cannot possibly be mapped.