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>

fs/zonefs/sysfs.c

@@ -15,11 +15,6 @@ struct zonefs_sysfs_attr {
 	ssize_t (*show)(struct zonefs_sb_info *sbi, char *buf);
 };
 
-static inline struct zonefs_sysfs_attr *to_attr(struct attribute *attr)
-{
-	return container_of(attr, struct zonefs_sysfs_attr, attr);
-}
-
 #define ZONEFS_SYSFS_ATTR_RO(name) \
 static struct zonefs_sysfs_attr zonefs_sysfs_attr_##name = __ATTR_RO(name)