kasan: docs: SLUB is the only remaining slab implementation

kasan: docs: SLUB is the only remaining slab implementation

We have only the SLUB implementation left in the kernel (referred to as
"slab").  Therefore, there is nothing special regarding KASAN modes when
it comes to the slab allocator anymore.

Drop the stale comment regarding differing SLUB vs. SLAB support.

Link: https://lkml.kernel.org/r/20260303120416.62580-1-david@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>