btrfs: Remove unused root var from relink_file_extents

Added in 38c227d8 ("Btrfs: snapshot-aware defrag") but subsequently
made redundant by 0b246afa ("btrfs: root->fs_info cleanup, add
fs_info convenience variables").
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -2772,12 +2772,10 @@ static void relink_file_extents(struct new_sa_defrag_extent *new)
 	struct sa_defrag_extent_backref *backref;
 	struct sa_defrag_extent_backref *prev = NULL;
 	struct inode *inode;
-	struct btrfs_root *root;
 	struct rb_node *node;
 	int ret;
 
 	inode = new->inode;
-	root = BTRFS_I(inode)->root;
 
 	path = btrfs_alloc_path();
 	if (!path)