btrfs: remove stray comment about SRCU

The subvol_srcu was removed in c75e8394 ("btrfs: kill the
subvol_srcu") years ago.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>