UPSTREAM: mm/ksm: Remove reuse_ksm_page()

Remove the function as the last reference has gone away with the do_wp_page()
changes.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 1a0cf26323c80e2f1c58fc04f15686de61bfab0c)

Bug: 176847924
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Change-Id: I70e5938a046d0fc449288ae46c83cb7c39d7de48