Btrfs: drop useless LIST_HEAD in merge_reloc_root

Drop LIST_HEAD where the variable it declares is never used.

The uses were removed in 3fd0a558 ("Btrfs: Metadata ENOSPC
handling for balance"), but not the declaration.

The semantic patch that fixes this problem is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@@
identifier x;
@@
- LIST_HEAD(x);
  ... when != x
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>