watchdog: kill unref/ref ops

ref/unref ops are not called at all so even marked them as deprecated
is misleading, we need to just drop the API.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>