This was added in commit 361048f5 ("Btrfs: fix full backref problem
when inserting shared block reference") to address a problem where we
hit the following BUG_ON() in alloc_reserved_tree_block

        if (node->type == BTRFS_SHARED_BLOCK_REF_KEY) {
                BUG_ON(!(flags & BTRFS_BLOCK_FLAG_FULL_BACKREF));

However this BUG_ON() is bogus, and was removed by previous commit:

  btrfs: remove bogus BUG_ON in alloc_reserved_tree_block

We no longer need to run delayed refs because of this, and can remove
this flushing here.
Signed-off-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>