thp: cleanup split_huge_page()

After one of bugfixes to freeze_page(), we don't have freezed pages in
rmap, therefore mapcount of all subpages of freezed THP is zero.  And we
have assert for that.

Let's drop code which deal with non-zero mapcount of subpages.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>