Btrfs: remove confusing tracepoint in btrfs_add_reserved_bytes

Here we're not releasing any space, but transferring bytes from
->bytes_may_use to ->bytes_reserved. The last change to the code in
commit 18513091 ("btrfs: update btrfs_space_info's
bytes_may_use timely") removed a conditional tracepoint and the logic
changed too but the tracepiont remained.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>