Commit [1] removed the need to use btrfs_async_submit_limit(), so
delete it.
[1]
 commit 
736cd52e0c720103f52ab9da47b6cc3af6b083f6
  Btrfs: remove nr_async_submits and async_submit_draining
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
 
        return 0;
 }
 
-unsigned long btrfs_async_submit_limit(struct btrfs_fs_info *info)
-{
-       unsigned long limit = min_t(unsigned long,
-                                   info->thread_pool_size,
-                                   info->fs_devices->open_devices);
-       return 256 * limit;
-}
-
 static void run_one_async_start(struct btrfs_work *work)
 {
        struct async_submit_bio *async;
 
                        u64 bio_offset, void *private_data,
                        extent_submit_bio_hook_t *submit_bio_start,
                        extent_submit_bio_hook_t *submit_bio_done);
-unsigned long btrfs_async_submit_limit(struct btrfs_fs_info *info);
 int btrfs_write_tree_block(struct extent_buffer *buf);
 void btrfs_wait_tree_block_writeback(struct extent_buffer *buf);
 int btrfs_init_log_root_tree(struct btrfs_trans_handle *trans,