mempool:Use default alignment inside of blockalign

can reduce memery usage

Signed-off-by: anjiahao <anjiahao@xiaomi.com>

mm/mempool/mempool_multiple.c

@@ -439,9 +439,6 @@ mempool_multiple_init(FAR const char *name,
       pools[i].priv = mpool;
       pools[i].alloc = mempool_multiple_alloc_callback;
       pools[i].free = mempool_multiple_free_callback;
-#if CONFIG_MM_BACKTRACE >= 0
-      pools[i].blockalign = mpool->minpoolsize;
-#endif
       ret = mempool_init(pools + i, name);
       if (ret < 0)
         {