btrfs: sysfs: drop unnecessary double logical negation in acl_show()

The IS_ENABLED() macro already guarantees the result will be a
suitable boolean return value ("1" for enabled, and "0" for disabled).
Thus, it seems that the "!!" used right before is unnecessary to force
the 0/1 values.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Neal Gompa <neal@gompa.dev>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>