mm/slab: remove mm/slab.c and slab_def.h

Remove the SLAB implementation. Update CREDITS.
Also update and properly sort the SLOB entry there.

RIP SLAB allocator (1996 - 2024)

Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: David Rientjes <rientjes@google.com>
Tested-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>