mm/slab: remove some unused functions

alternate_node_alloc and ____cache_alloc_node are always called when
CONFIG_NUMA. So we can remove the unused !CONFIG_NUMA variant. Also
forward declaration for alternate_node_alloc is unnecessary. Remove
it too.

[ vbabka@suse.cz: move ____cache_alloc_node() declaration closer to
  its callers ]

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/20220322091421.25285-1-linmiaohe@huawei.com