mm: Remove shrink_page

By previous patch, shrink_page_list can handle pages from
multiple zone so let's remove shrink_page.

Change-Id: I3526377aa6ee6142b8f3ec63396e7ada1e442505
Signed-off-by: Minchan Kim <minchan@kernel.org>
Patch-mainline: linux-mm @ 22 Apr 2013 17:45:03
[vinmenon@codeaurora.org: trivial merge conflict fixes]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>