vm_page: Remove vm_page_sbusy() and vm_page_xbusy()

They are unused today and cannot be safely used in the face of unlocked
lookup, in which pages may be busied without the object lock held.

Obtained from: jeff (object_concurrency patches)
Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D32948