slab: remove kmem_cache_create_rcu()

slab: remove kmem_cache_create_rcu()

Now that we have ported all users of kmem_cache_create_rcu() to struct
kmem_cache_args the function is unused and can be removed.

Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>