mm/kasan: remove unnecessary CONFIG_KASAN option

mm/kasan: remove unnecessary CONFIG_KASAN option

In mm/Makefile has:

  obj-$(CONFIG_KASAN)     += kasan/

So that we don't need 'obj-$(CONFIG_KASAN) :=' in mm/kasan/Makefile,
delete it from mm/kasan/Makefile.

Link: https://lkml.kernel.org/r/20220221065421.20689-1-tangmeng@uniontech.com
Signed-off-by: tangmeng <tangmeng@uniontech.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>