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: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>