arm: bcm283x: remove unused empty hw_watchdog_disable

This empty stub was originally added as one branch of an #ifdef in
commit 45a6d231 (bcm2835_wdt: support for the BCM2835/2836
watchdog). That incarnation of the rpi watchdog driver was later
removed in c7adc0b5 (watchdog: bcm2835_wdt: Remove unused BCM283x
watchdog driver and its references), but this now unused stub was left
behind. The later (re-)added rpi watchdog driver does not define a
hw_watchdog_disable() function, as that is properly integrated in the
watchdog framework.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>