kasan, arm64: remove redundant ARCH_SLAB_MINALIGN define

Defining ARCH_SLAB_MINALIGN in arch/arm64/include/asm/cache.h when KASAN
is off is not needed, as it is defined in defined in include/linux/slab.h
as ifndef.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>