Watchdog: remove outdated comment

The lock field doesn't exist in watchdog_device structure.
It was added by commit f4e9c82f64 ("watchdog: Add Locking support")
and removed by commit b4ffb19098
("watchdog: Separate and maintain variables based on variable lifetime")

Signed-off-by: Hardik Singh Rathore <hardiksingh.k@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

include/linux/watchdog.h

@@ -90,9 +90,6 @@ struct watchdog_ops {
  *
  * The driver-data field may not be accessed directly. It must be accessed
  * via the watchdog_set_drvdata and watchdog_get_drvdata helpers.
- *
- * The lock field is for watchdog core internal use only and should not be
- * touched.
  */
 struct watchdog_device {
 	int id;