kernel: remove CONFIG_USE_GENERIC_SMP_HELPERS cleanly

Remove CONFIG_USE_GENERIC_SMP_HELPERS left by commit 0a06ff06
("kernel: remove CONFIG_USE_GENERIC_SMP_HELPERS").

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>