btrfs: remove unused function btrfs_async_submit_limit()

Commit [1] removed the need to use btrfs_async_submit_limit(), so
delete it.

[1]
 commit 736cd52e
  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>

fs/btrfs/disk-io.c

@@ -710,14 +710,6 @@ blk_status_t btrfs_bio_wq_end_io(struct btrfs_fs_info *info, struct bio *bio,
 	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;

fs/btrfs/disk-io.h

@@ -133,7 +133,6 @@ blk_status_t btrfs_wq_submit_bio(struct btrfs_fs_info *fs_info, struct bio *bio,
 			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,