md/raid1: remove struct pool_info and related code

The struct pool_info was originally introduced mainly to support reshape
operations, serving as a parameter for mempool_init() when raid_disks
changes. Now that mempool_create_kmalloc_pool() is sufficient for this
purpose, struct pool_info and its related code are no longer needed.

Remove struct pool_info and all associated code.

Signed-off-by: Wang Jinchao <wangjinchao600@gmail.com>
Link: https://lore.kernel.org/linux-raid/20250707012711.376844-3-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>