zonefs: Remove to_attr() helper function

to_attr() in zonefs sysfs code is unused, which it causes a warning when
compiling with clang and W=1. Delete it to prevent the warning.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>