btrfs: drop stale reference to volume_mutex

Commit dccdb07b ("btrfs: kill btrfs_fs_info::volume_mutex") removed
the last use of the volume_mutex, forgetting to update the comment.
Signed-off-by: NAnand Jain <anand.jain@oracle.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -280,7 +280,6 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
  * ============
  *
  * uuid_mutex
- *   volume_mutex
  *   device_list_mutex
  *     chunk_mutex
  *   balance_mutex