include/linux/mm.h: remove unneeded ifdef

The check code for CONFIG_SWAP is redundant, because there is a
non-CONFIG_SWAP version for PageSwapCache() which just returns 0.

Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>