mm/slob: remove slob.c

Remove the SLOB implementation.

RIP SLOB allocator (2006 - 2023)
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>