include/linux/swap.h: delete meaningless __add_to_swap_cache() declaration

Since commit 8d93b41c ("mm: Convert add_to_swap_cache to XArray"),
__add_to_swap_cache and add_to_swap_cache are combined into one
function.  There is no __add_to_swap_cache() anymore.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Link: http://lkml.kernel.org/r/1590810326-2493-1-git-send-email-linmiaohe@huawei.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>