board: rpi: remove leftover CONFIG_HW_WATCHDOG block

This was added in commit 45a6d231 (bcm2835_wdt: support for the
BCM2835/2836 watchdog), which did do 'select HW_WATCHDOG'. That
incarnation of the watchdog driver later got removed in
c7adc0b5 (watchdog: bcm2835_wdt: Remove unused BCM283x watchdog
driver and its references), but this block was left behind.

Another rpi watchdog driver has since been added, but that does not
select HW_WATCHDOG, so this remains dead and unused. Remove it.

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