watchdog: iTCO_wdt: Remove unused hooks

As the only user of iTCO_vendor_pre_keepalive and
iTCO_vendor_pre_set_heartbeat has just been removed, we can delete
these 2 hooks.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Martin Wilck <mwilck@suse.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>